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.

packages/client/test/inbox.test.ts

@@ -1,5 +1,6 @@
 import { test, expect, mock, beforeEach, afterAll } from "bun:test";
 import { post, read, materialize, watch } from "../src/inbox";
+import { walletInbox, resetRegistryCache } from "../src/store-registry";
 import type { Deposit } from "../src/inbox";
 import {
   configure,
@@ -147,7 +148,8 @@ function makeFakeNg() {
 }
 
 const SESSION: RegistrySession = { sessionId: "sid-1", privateStoreId: "PRIV" };
-const TARGET = "did:ng:o:host-inbox";
+/** Resolved per test: an inbox BELONGS to a wallet, and only its owner may read it. */
+let TARGET: `did:ng:${string}`;
 
 function inject() {
   const ng = makeFakeNg();
@@ -159,15 +161,20 @@ function inject() {
 }
 
 let fake: ReturnType<typeof makeFakeNg>;
-beforeEach(() => {
+beforeEach(async () => {
   fake = inject();
+  resetRegistryCache();
+  setCurrentUser("alice");
+  TARGET = await walletInbox("alice");
 });
 
 test("post writes via the real injected ng.sparql_update (not makeNg), scoped to the inbox", async () => {
   setCurrentUser("alice"); // `from` is bound to the current identity
+  // Count from HERE: resolving this wallet's own inbox already wrote to the shim.
+  const before = fake.sparql_update.mock.calls.length;
   await post(TARGET, { from: "alice", payload: { kind: "join" }, ts: 100 });
-  expect(fake.sparql_update).toHaveBeenCalledTimes(1);
-  const call = fake.sparql_update.mock.calls[0]!;
+  expect(fake.sparql_update.mock.calls.length).toBe(before + 1);
+  const call = fake.sparql_update.mock.calls[before]!;
   expect(call[0]).toBe("sid-1"); // sessionId from the injected session
   expect(call[2]).toBe(TARGET); // anchored to the target inbox
   // The write targets the anchored DEFAULT graph — NO explicit `GRAPH <…>`