Refactor read filter from per-item grant to per-document ReadCap

Model the read filter on NextGraph's real ReadCap mechanism instead of an
invented per-item grant. Verified in nextgraph-rs: there is no Document type
(document = repo); a store is a container repo referencing other repos by RDF
overlay; holding a store's cap does NOT grant the repos it contains (each repo
needs its own cap; no read-cap inheritance). So the access unit is the
DOCUMENT = an item's `@graph`, never the item.

- caps.ts: CapRegistry (read/write caps per document NURI + public docs;
  open/grantRead/grantWrite/makePublic/canRead/canWrite/governsRead/
  hasReadPolicy). Replaces access.ts (Grant).
- read-filter.ts: filter keeps an item iff its `@graph` document is readable
  (held cap or public); items with no `@graph` or in an ungoverned document are
  kept. No injected grantOf — the filter reads `@graph` and consults the
  registry (automatic, domain-agnostic).
- polyfill.ts: getCaps()/resetCaps() replace setGrantOf/getGrantOf; useShape
  filters only when caps.hasReadPolicy() (else passthrough, no regression).
- tests: caps.test.ts (6) + read-filter.test.ts (4), incl. no-inheritance
  between documents. 10 pass; tsc rc=0.

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

packages/client/test/caps.test.ts

@@ -0,0 +1,50 @@
+import { test, expect } from "bun:test";
+import { CapRegistry } from "../src/caps";
+
+test("public documents are readable by anyone, even anonymous", () => {
+  const caps = new CapRegistry();
+  caps.open("did:ng:o:pub", "public", "alice");
+  expect(caps.canRead("did:ng:o:pub", null)).toBe(true);
+  expect(caps.canRead("did:ng:o:pub", "bob")).toBe(true);
+});
+
+test("protected documents: owner + explicitly granted principals only", () => {
+  const caps = new CapRegistry();
+  caps.open("did:ng:o:prot", "protected", "alice");
+  expect(caps.canRead("did:ng:o:prot", "alice")).toBe(true);
+  expect(caps.canRead("did:ng:o:prot", "bob")).toBe(false);
+  caps.grantRead("did:ng:o:prot", "bob"); // bob becomes a connection of alice
+  expect(caps.canRead("did:ng:o:prot", "bob")).toBe(true);
+});
+
+test("private documents: owner only", () => {
+  const caps = new CapRegistry();
+  caps.open("did:ng:o:priv", "private", "alice");
+  expect(caps.canRead("did:ng:o:priv", "alice")).toBe(true);
+  expect(caps.canRead("did:ng:o:priv", "bob")).toBe(false);
+  expect(caps.canRead("did:ng:o:priv", null)).toBe(false);
+});
+
+test("write is restricted to write-cap holders; the creator always holds it", () => {
+  const caps = new CapRegistry();
+  caps.open("did:ng:o:pub", "public", "alice");
+  expect(caps.canWrite("did:ng:o:pub", "alice")).toBe(true);
+  expect(caps.canWrite("did:ng:o:pub", "bob")).toBe(false);
+  expect(caps.canWrite("did:ng:o:pub", null)).toBe(false);
+});
+
+test("holding a document's cap does NOT grant another document (no inheritance)", () => {
+  const caps = new CapRegistry();
+  caps.grantRead("did:ng:o:doc1", "alice");
+  expect(caps.canRead("did:ng:o:doc1", "alice")).toBe(true);
+  expect(caps.canRead("did:ng:o:doc2", "alice")).toBe(false); // separate repo, separate cap
+});
+
+test("governsRead / hasReadPolicy distinguish governed from ungoverned documents", () => {
+  const caps = new CapRegistry();
+  expect(caps.hasReadPolicy()).toBe(false);
+  caps.grantRead("did:ng:o:doc1", "alice");
+  expect(caps.hasReadPolicy()).toBe(true);
+  expect(caps.governsRead("did:ng:o:doc1")).toBe(true);
+  expect(caps.governsRead("did:ng:o:unknown")).toBe(false); // not declared → not enforced
+});