Keep sign-ups unlinkable on ActivityPods, and land the pending coherence-pass work

Participation privacy (decision_2026-10-29_participation-privacy-mechanism):
a sign-up is a private Join naming the meeting point, addressed to nobody,
handled by the backend on emission. Attendee lists hold member URNs hashed
from (WebID, event, a secret kept in the participant's profile), so counts
stay public and only contacts recognise anyone; the creator has no read
right and is notified anonymously. Each person's participations collection,
created by the backend at the first sign-up, is readable by their /contacts
WAC group only, never world-readable. The member URN is sticky per
(meeting point, participant) and every collection write is verified by
reading back, so a refused write leaves the act unrecorded and retryable.
The EventRef mechanism and its browser-side provisioning are gone.

The backend now runs with WebACL on, as the reference apps do, on its own
secure datasets festipod / settings-festipod instead of the pod provider's
settings dataset; pod-notifications needs it. The compose header and the
deployment leaf say how to reset a stack whose datasets predate this.

The rights matrix covers the new mechanism live (148 hard lines), including
a forged contact Accept between throwaway accounts: the stranger enters the
contacts list (platform defect, reported upstream) but is refused the profile
and the participations. Tests may attack as long as they never break the dev
environment (rule_tests-never-break-the-dev-environment).

Also lands the coherence-pass work that was still uncommitted: contacts as
the Pod's social graph and invite links, image upload to the Pod, the
meeting-point form, profile completion gate, and the doctrine for them.
Superseded briefs and decisions are deleted rather than annotated.

.project/handoff/refine-provider-abstraction.md

@@ -87,20 +87,15 @@ The reconstruction is the specification. Its source was read **once, deliberatel
 - `types` takes precedence over `shapeTreeUri` when both are set — the **opposite** of what the configuration type documents.
 - `custom` is not implemented, so there is **no escape hatch through Refine** for non-CRUD operations.
 - Collections are POSTable LDP resources (`/as/collection`) with `semapps:dereferenceItems` — *"all items will be dereferenced (if permissions allow)"*. That is how `event.participations` can come back as full objects for a reader who is allowed, and as URIs otherwise, in one read. Items are added/removed by SparqlPatch on `as:items`.
-- `acl:Append` exists among the WAC modes and is really used in app access needs. **Whether it lets a third party insert an item without being able to remove one is NOT VERIFIED** — enforcement lives in `@semapps/webacl`, absent from the checkout. The structure does not depend on the answer: if it works that way the participant appends directly; otherwise the route is `apods:PostOutbox` to the owner's inbox, which is exactly the NextGraph shape, and only latency changes.
 - An app declares `accessNeeds` (`shapeTreeUri` + `accessMode`) plus special needs: `apods:CreateCollection`, `apods:CreateWacGroup`, `apods:PostOutbox`, `apods:QuerySparqlEndpoint`, `apods:UpdateWebId`, inbox/outbox reads.
 - **Backend-less apps are supported**: `onlineBackend` is false only when the app description document at the `clientId` URI is not fetchable. A static file suffices.
-- A custom ontology prefix **must be registered at prefix.cc** (the Pod provider requires every ontology to have a known prefix) — this concerns Festipod's `fp:`. Shape trees must sit at a public URL, one per entity, and **cannot reference other shapes** (no sharing cascade).
+- A Festipod **class** with its own shape tree needs `fp:` registered at prefix.cc (the pod registers the ontology of every class it creates a data registration for, and refuses an unknown prefix); shape trees must sit at a public URL, one per entity, and **cannot reference other shapes** (no sharing cascade). A Festipod **predicate** on an existing resource does not: verified 2026-10-29 on a profile with the owner's token (app-token path to confirm live).
 
-**Four contributions this points to** (we may contribute to these providers): `getMany` must signal the unreadable instead of swallowing it; an append path into a collection one does not own; collection support as a resource with `dereferenceItems`; and a teardown for the auth provider's timer.
+**Three contributions this points to** (we may contribute to these providers): `getMany` must signal the unreadable instead of swallowing it; collection support as a resource with `dereferenceItems`; and a teardown for the auth provider's timer.
 
-## The deduplication question — answered, and still open
+## Participation privacy and deduplication — settled
 
-The attendance model (`brief_2026-07-20_attendance-set-model`, PO 2026-07-27) requires the creator to tell two references from the *same* person apart from two from different people **without knowing who**, or the count is not a count of people and a participant can inflate it.
-
-**Checked on this base: not solved.** A different problem is, and well. `materializeAttendance` (`src/shared/data/registration.ts`) keys its active set by a **per-deposit** `uid` minted from time plus randomness, so a broker re-sync replaying the same deposits converges — that is **replay idempotence**, not person identity. The count counts **deposits**: the same person joining twice counts twice. Further: the **plaintext `userId` travels in the deposit** and the cancellation fallback depends on it (a leave without a known `regUid` cancels the earliest still-active join for that `userId`), so the creator *does* know who and the anonymity the model rests on does not hold. Participations are in the **protected** scope rather than public, and withdrawal is still a deletion. And the gap was **never raised** through the channel meant for it.
-
-So both platforms count deposits rather than people, and the gap sits in **neither Refine provider** — polyfill on one side, server on the other. ActivityPods could arbitrate server-side because the server sees everything.
+Settled 2026-10-29: `.project/concepts/functional-domain/decision_2026-10-29_participation-privacy-mechanism.md` — a private Join handled by the backend on emission, attendee lists of per-(person, event) member URNs (distinct-person counts, no identity), recognition by contacts only, a contacts-readable participations collection per person.
 
 ## Problems hit, and how they turned out
 
@@ -148,7 +143,7 @@ What was missing for Festipod's premise — discover other users' public events
 
 - **Attendee lists stay world-readable for now** (2026-10-12) — an explicit deviation from the domain doctrine, where participation is in the protected scope (`functional-domain`, `knowledge_data-scopes-and-discovery`): anonymous readers of a public meeting point's attendee collection see its members' WebIDs, which embed usernames. The matrix tracks it as a KNOWN-DEVIATION line (printed, non-fatal, flags loudly the day it changes). Moving participation to the protected scope is a later task.
 - **A revoked app logs the user out** (2026-10-12): picker with a notice, never sent back to consent.
-- **Defensive testing only** (2026-10-13): no automated check sends a forged or spoofed activity; such checks are MANUAL-CHECK lines (below).
+- **Tests never break the dev environment** (user rule 2026-10-30, concept `bdd-testing`): attacks, forged or spoofed activities included, are allowed between the throwaway accounts a run creates and deletes.
 - **Trusted local network** (2026-10-13): no key rotation after the exposure below; it is recorded as a risk. Until `88f23b7` the Festipod backend served, anonymously and on all interfaces (reached from the LAN), an open `/sparql` over its `settings` dataset — the app actor's private key, the pod provider's password hashes and emails (the dataset is shared with the pod provider locally), webhook URLs, catalog snapshots — and the private key again at `/key` through the LDP catch-all. Fuseki still listens on all interfaces with `admin/admin`.
 
 **MANUAL checks** — kept out of the automated matrix because they need a crafted request; each is covered by backend unit tests and owes one manual run by a human: (1) a Delete posted to a user's OWN outbox that embeds a Tombstone of ANOTHER user's event must not de-index it (`catalog.service` ownership, `emitted-activity` re-read); (2) a forged pod notification (webhook) must not publish a private event, grant or revoke a read, or list/de-list anything; (3) a crafted delivery to `POST /as/actor/app/inbox` or `/.webhooks/<id>`, and any write outside them, must change nothing (`public-surface`, `emitted-activity`).
@@ -176,7 +171,7 @@ What was missing for Festipod's premise — discover other users' public events
 - Adding `## Needs` to the polyfill declaration is where the per-person discriminator belongs. It is **outward-facing** (the provider reads it on its next pull), so it awaits the user's go.
 - `acl:Append` behaviour, as above.
 - The router's prefix stripping, with the second backend.
-- The four provider contributions.
+- The three provider contributions.
 - Wiring the seam: DONE 2026-10-09 (T04 tree, tip `f8c2e3b`) — `FestipodDataContext` runs over Refine with the ActivityPods backend as the registered default; the join→collection loop is built but `@suspendu` until the app is installed on a pod (consent screen). Current state: `## Current gate state` above. The app-install blocker is CLEARED end-to-end 2026-10-12 (tips `3aa9131` → this state): the login → Solid-OIDC → consent → registration → app loop completes on the aligned stack, verified by an automated Playwright probe (fresh user: picker → login → consent → `/home` authenticated → event created → meeting point attached). The chain of causes, in the order they surfaced: (1) the pod provider's OIDC adapter resolves an unknown client_id by fetching it and requires a Solid-OIDC Client ID Document — `/app` serves it when the request carries the `JsonLdContext` header, the actor otherwise; (2) the platform stack has to run a coherent version — the app packages are pinned at npm latest (`@activitypods/app` 2.3.2, refine-providers 0.2.0) and the pod provider runs the release tag v2.3.12 (fresh pull + full data reset + the pod frontend needing `VITE_CONFIG_URL` in dev and redis remapped to :6390 per the cookbook); (3) on a cold start the actor's `interop:hasAccessNeedGroup` link races the access-need-group creation and is never attached — an idempotent heal in `backend/services/app.service.js` `started()` patches it when missing; (4) the auth provider's `registerApp` discovers the AppRegistration via a `Link: rel="interop:registeredAgent"` header the pod only returns when the requester's token identity (azp = the configured client_id) equals the registration's `registeredAgent`, which the pod stores under the app ACTOR uri — so the Solid-OIDC client_id IS the actor uri (`${backendUrl}/as/actor/app`), and `/as/actor/app` serves the actor / the Client ID Document by the same header trick through a standalone `client-id-doc` service (routing it to `app.get` deadlocked boot: the app service's own `awaitCreateComplete` polls the actor over HTTP while the service is still starting). The pinned contract carries a dated re-baseline section recording the registerApp/getAppStatus findings (authorized one-time re-read). The verify-compat shim was removed once the aligned stack made it redundant. A revoked app is now detected client-side (`4fc856f`, `d4fcb7e` — see `## T05`). The join→collection scenarios are re-enabled and green (5/5 smoke), and the full authenticated UI flow is banked as a durable gate: `bun run test:e2e-flow` (scripts/e2e-login-flow.ts — picker → login → consent → authenticated → event → meeting point, fresh user, benign-4xx classification, non-zero exit on any fatal ≥400). Platform fragility observed while un-suspending: hand-crafting an `apods:attendees` collection (instead of letting the backend attach it) crashed the festipod backend with an uncaught `Invalid URL` in `@activitypods/app` `pod-collections-handler.getGroupSlugFromCollectionUri(undefined)` — the scenario now reads the attached collection rather than building it; the crash path itself is platform-side.
 - Both usage declarations are named `usage_festipod.md`, so the bare stem is ambiguous as a wikilink — refer to either by folder path.
 - Driving `getAppStatus()`: DONE (`4fc856f`, `d4fcb7e`). Festipod relies on its `installed` member, which the reconstruction does not pin — recorded in the activitypods-providers usage declaration (constraint 9, need 3), together with its reliance on `create` sending `to` unchanged (constraint 10).