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/src/index.ts

@@ -23,6 +23,12 @@ export type { Connections, IsolationAccessors } from "./isolation";
 export * as accounts from "./accounts";
 export type { AccountStorage } from "./accounts";
 
+// SPARQL injection-safety helpers — so the app can reuse the same escaping /
+// validation when it builds SPARQL by interpolation. `escapeLiteral` for string
+// literals, `escapeIri` to embed untrusted values in an IRI, `assertNuri` to
+// validate trusted-shaped NURIs before embedding them in an IRI.
+export { escapeLiteral, escapeIri, assertNuri } from "./sparql";
+
 // SDK type re-exports — so the app imports these from @ng-eventually/client too,
 // not from @ng-org. `export type` is ERASED at build, so this adds NO runtime
 // @ng-org import to the lib (no risk of a duplicate SDK copy in the bundle).