Align the cap emulation on NextGraph's model, and confine it to a virtual user

Two batches, verified against nextgraph-rs throughout.

P1a — the capability surface. Reading was an ACL (Map<doc, Set<principal>>),
the exact inversion of key possession. It is now possession: `capFor(nuri)` is
the only question, there is no principal parameter anywhere, and nothing turns
a bare reference into a cap. Sharing is `shareCap(cap, toInbox)`, a Link
deposit; receiving needs no operation. `Nuri` and `ReadCap` are template
literal types, so passing a bare reference where a cap belongs is a compile
error, with runtime guards behind it for JavaScript callers.

The virtual user boundary. Every access function is now confined to the
connected user, through two rules on one criterion (possession), implemented in
two places so a lapse in either is caught by the other: authorization at the
passage points, and "do not even attempt" at the callers. The polyfill's own
machinery moved to physical.ts — unguarded, never exported — which replaced an
exemption list: the machinery no longer gets waved through the guard, it calls
something the guard never saw.

Removed, as emulating capabilities the target does not have:
- discovery.ts and its global index. There is no discovery in NextGraph; you
  follow links. It also pooled user data across wallets.
- the cross-account fan-out (listEntityDocs, resolveReadGraphs, allAccounts,
  loadShim), which was cross-user enumeration by construction.
- resolveInboxAnchor, a single inbox common to every user.

Caps are now stored where NextGraph stores them, and read back rather than
recomputed: AddRepo on the store's Store branch for documents a user creates,
AddLink on its User branch for caps received. Inboxes belong to someone — the
user's own, plus one per document — and connecting a user drains them all;
that is the library's job, not the app's.

Corrections worth recording: a ReadCap is `r:`, not `:k:` (reported by
NextGraph's developer, verified in BlockRef::readcap_nuri); received caps DO
have a register (AddLink), contrary to what this repo's notes claimed; and
"wallet" upstream means keyring — what owns three stores is a user, so the
vocabulary follows.

The cap value is the constant OK: the only question the emulation answers is
whether a cap is held. P1b replaces that one constant with a real key.

After this the shape is right and the isolation is still fake. Nothing here may
be described as anonymous or private.

docs/decisions/discovery-model.md

@@ -1,6 +1,23 @@
 # ADR — Discovery mechanism (inbox-fed index, fan-out)
 
-**Date:** 2026-06-16 · **Status:** mechanism accepted; target owner undecided.
+> ## SUPERSEDED — 2026-07-30. The premise does not hold.
+>
+> **There is no discovery in NextGraph. You cannot discover; you can only follow links** (PO, 2026-07-30 — the principle is documented in [`../readcap-and-nuri-model.md`](../readcap-and-nuri-model.md) §4ter-bis). Publishing is two acts: place the data in your public store, **and** circulate the link — into inboxes, or into somewhere already reachable by the people concerned. It is seen only by those who received the link. This is a foundation of local-first, not a gap to be filled.
+>
+> A global index therefore fails on **two independent counts**:
+>
+> 1. it emulates a capability the target will never have — teaching consumers a model that does not exist, which is the one failure mode this library exists to prevent;
+> 2. it is **data common to several users/wallets**, and nothing may be common — only indexing mechanisms that make the virtual users work (the shim qualifies; a shared index of user announcements does not).
+>
+> This ADR already recorded the first half of that verdict — *"a dedicated service with its own wallet sharing a freely-readable index is not a NextGraph shape"*, resting on a singleton-app path *"not implemented, uncertain"*. That reservation is now the conclusion.
+>
+> **`discovery.ts` and its tests were removed on 2026-07-30**, along with `watchShape`'s public-scope fold and `INDEX_ACCOUNT`. See [`../briefs/2026-07-30-virtual-wallet-boundary.md`](../briefs/2026-07-30-virtual-wallet-boundary.md).
+>
+> What survives, and is worth keeping from the text below: the **3-stage frame** (`discovery → synchronization → query`) is still exactly right, with stage 1 re-read as *"a link reached you"* rather than *"you consulted an index"*. You still cannot query what you have not synchronized, and you still do not synchronize what nobody gave you. The **inbox** is what feeds stage 1 — which makes it the bootstrap of the whole reachability graph, not a side feature.
+>
+> Kept in full below as a record of what was built and why, and of the reasoning that has to be re-read through the correction above.
+
+**Date:** 2026-06-16 · **Status:** SUPERSEDED 2026-07-30 (see the block above). *Originally: mechanism accepted; target owner undecided.*
 Ported here for the discovery mechanism it defines — the piece this lib
 realizes (`inbox.ts` post/materialize/watch; `store-registry.ts` fan-out). The
 product intent (what a consumer application *should* surface) is the consumer