feat: un document en store public sert son ReadCap, une référence nue suffit

Le modèle amont est explicite dans `PublicRepoLinkV0` : le lien ne porte AUCUN
`read_cap`, et son commentaire dit pourquoi — *"The latest ReadCap of the branch
will be downloaded from the outerOverlay, if the peer brokers listed below allow
it […] the public site are served differently by brokers"*
(engine/net/src/types.rs:5098). La clé n'est pas remise par un émetteur : elle est
donnée par le réseau à qui la demande, parce que le broker a épinglé l'overlay
externe (`expose_outer`).

La bibliothèque refusait jusqu'ici la forme sans cap quel que soit le store. Sûr
dans le bon sens, mais une application ne pouvait pas exprimer « fais circuler, la
référence suffit » — le seul acte que le modèle rend gratuit — et son unique
contournement était de distribuer la clé, ce qui détruit la confidentialité
composable.

`emulated-verifier/public-store.ts` émule le mécanisme SANS toucher à la garde. La
possession reste l'unique critère : un document public est lisible non par exception
mais parce que son cap est *obtenable*. Chaque porte de lecture demande d'abord
(`readUnion`, `docs.sparqlQuery`, `ensureRepoOpen`, `documentInboxAddress`), puis le
chemin ordinaire s'applique.

Lire n'est pas écrire. Ce que le store sert est un droit de LECTURE :
`learnFromPublicStore` le classe à part et `assertMayWrite` refuse l'écriture
dessus. Sans cela une référence nue achetait une écriture, ce qu'aucun store amont
n'accorde.

Autres conséquences :

- `recordInPublicStore` (marquer + frapper) devient `markInPublicStore` (marquer).
  Frapper un second cap à côté de celui qu'on vient de télécharger donnerait deux
  clés différentes le jour où la constante devient un secret.
- `hasCap` quitte la porte polyfill : il se lisait « ai-je le droit de lire ceci ? »
  et un document public y répondait `false` jusqu'à ce qu'on demande son cap. Aucun
  appelant hors des tests.
- Les tests cross-user ne font plus traverser de cap par une variable JS : Bob
  n'obtient que la référence nue, comme une vraie application.

Écarts documentés plutôt que masqués : le pari sur un modèle DÉCLARÉ (`expose_outer`
est câblé à `false` côté client et `ExtTopicSyncReq` est `unimplemented!()`), la
découverte limitée à ce qu'on sait déjà nommer, `useShape` qui n'a pas d'await à
dépenser, et l'absence de `locator`.

179 tests unitaires, e2e 42/42 contre le broker en ligne.

docs/document-links.md

@@ -50,11 +50,15 @@ What follows for a public document, and is easy to get wrong: **there is nothing
 
 ### The polyfill's public emulation, read against this
 
-Verified in this repo: publication **mints a cap** (`publishRepoLink` → `mint`, `packages/client/src/emulated-verifier/caps.ts:192-196`), the `published` set is consulted by no read path (emitter-side guard only), and the possession filter gates public documents exactly like private ones. Against the target *under the emulation's own topology* (one broker, so join-reachability is trivially satisfied): upstream, whoever can name a public document reads it; the polyfill refuses the cap-less form for everyone. That is **over-strict, not inverted** — it under-grants and never over-grants, and "circulate the link" remains the valid currency at migration. But three deltas deserve to stay visible wherever the public emulation is documented:
+**Rewritten 2026-08-06**, when the emulation changed. It used to refuse a cap-less reference for every scope, which was over-strict in the safe direction but left an application unable to express *"circulate widely, the reference is enough"* — the one act the model makes cheap. `emulated-verifier/public-store.ts` now emulates the declared mechanism: a document in a public store exposes its ReadCap, and any reader's first door fetches it. Possession still decides everything; what changed is that for a public document the cap is **obtainable** instead of having to be handed over.
 
-1. A **cap-less reference to a public document** embedded in reachable content will resolve upstream (once public serving is wired) and does not resolve here.
-2. **Per-reader semantics for public documents** must not be inferred from the emulation's cap-per-holder bookkeeping — upstream has none.
-3. The `:r:` segment inside a link to a *public* document is emulation detail: upstream's public link carries no key material. Harmless as long as the value is opaque to the consumer — which is the contract to enforce.
+**This aligns on a DECLARED model, not on current behaviour, and the difference is worth stating.** What is read in source: `PublicRepoLinkV0` carries no `read_cap` and its comment says *"The latest ReadCap of the branch will be downloaded from the outerOverlay, if the peer brokers listed below allow it […] the public site are served differently by brokers"* (`engine/net/src/types.rs:5098-5124`); the broker's `expose_outer` plumbing exists (`engine/broker/src/server_storage/core/overlay.rs:103-133`). What is *not* wired today, per the inventory above: the client hard-codes `expose_outer: false` in both `PinRepo` constructors, and `ExtTopicSyncReq` — the anonymous branch-sync such a link needs — is `unimplemented!()`. So the emulation follows the model the engine declares, in a place the engine does not yet serve. That is the intended posture for this library (an absent implementation says nothing about what the target will do), and it is a bet, named here as one.
+
+Three things that remain true and must stay visible wherever the public emulation is documented:
+
+1. **Per-reader semantics for public documents** must not be inferred from the cap-per-holder bookkeeping — upstream has none. No grant, no per-reader revocation, no audience list.
+2. The `:r:` segment inside anything naming a *public* document is emulation detail: upstream's public link carries no key material, because the key is fetched. Harmless as long as the value stays opaque to the consumer — which is the contract to enforce.
+3. **Reading is not writing.** The cap a public store serves grants reading only; `caps.learnFromPublicStore` files it apart and `docs.sparqlUpdate` refuses a write on it. Upstream a public store never makes a repo world-writable — writing needs the write cap, and `verify_permission` fires on WRITE only.
 
 The surface consequence: the *act* — obtain a link, circulate it — is the same for both scopes upstream (`NgLinkV0` is one enum over both), so one producing function covering both is target-shaped; what differs is the semantics attached to the **value** (durability, revocability, the absence of per-reader anything), and that belongs in documentation, not in a second function.