feat(client): inbox mechanism, write-guard, SPARQL injection hardening

Polyfill capabilities landed for Festipod's T02 features (all generic,
zero-domain — the consumer injects the domain).

- inbox: implement the previously-stubbed namespace. post(target,{from?,
  payload,ts?}) deposits {from,payload,ts} as RDF via docs.sparqlUpdate (the
  real injected ng, never makeNg); read/materialize + watch emulate the curator
  in-lib (deposits read via docs.sparqlQuery). `from` optional = anonymity.
- write-guard: caps.hasWritePolicy() + ng-proxy.sparql_update rejects when the
  target doc is under a write policy and the current user lacks its write cap;
  passthrough otherwise (no regression). Read-cap registry unchanged.
- sparql.ts (new): escapeLiteral / escapeIri / assertNuri, exported from index.
  store-registry now escapes every literal and validates/encodes every IRI-
  position value — closes a SPARQL-injection hole where an untrusted username
  could inject triples into the shim (the account→doc-NURI trust root).

Tests: inbox, sparql (incl. injection), ng-proxy write-guard, isolation-active.
68 tests pass; tsc --noEmit rc=0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/client/test/caps.test.ts

@@ -48,3 +48,17 @@ test("governsRead / hasReadPolicy distinguish governed from ungoverned documents
   expect(caps.governsRead("did:ng:o:doc1")).toBe(true);
   expect(caps.governsRead("did:ng:o:unknown")).toBe(false); // not declared → not enforced
 });
+
+test("governsWrite / hasWritePolicy distinguish governed from ungoverned documents", () => {
+  const caps = new CapRegistry();
+  expect(caps.hasWritePolicy()).toBe(false);
+  caps.open("did:ng:o:doc1", "private", "alice"); // owner gets the write cap
+  expect(caps.hasWritePolicy()).toBe(true);
+  expect(caps.governsWrite("did:ng:o:doc1")).toBe(true);
+  expect(caps.governsWrite("did:ng:o:unknown")).toBe(false); // not declared → not enforced
+  // A public doc grants read to all but its write cap is still owner-only.
+  const pub = new CapRegistry();
+  pub.open("did:ng:o:pub", "public", "alice");
+  expect(pub.hasWritePolicy()).toBe(true);
+  expect(pub.governsWrite("did:ng:o:pub")).toBe(true);
+});