Reject registerTool on already-aborted signal (Chrome 152); sweep stale navigator.modelContext references

[kashishhora]

What changed

Chrome 152.0.7943.0 shipped two relevant changes (per the Chrome WebMCP team's EPP announcement and verified against Chromium source at the 152.0.7943.0 tag):

1. navigator.modelContext is now fully removed (deprecated since 150; this library migrated to document.modelContext in 0.2.0 / Migrate WebMCP surface to document.modelContext (Promise registration, single-arg execute) #23).
2. Native registerTool now rejects with the signal's abort reason when handed an already-aborted AbortSignal — the behavior specified by webmcp spec PR #202 (merged June 17), which had not shipped as of Chrome 151. Our docs carried a "revisit if native changes" caveat for exactly this.

Behavior change

• src/polyfill/registry.ts — registerTool with an already-aborted signal now rejects with signal.reason instead of resolving as a no-op. The check stays after validation, matching the spec's step order (validation errors take precedence).
• src/polyfill/__tests__/registry.test.ts — test flipped to assert rejection with the exact abort reason.
• useMcpTool needs no change: it never passes a pre-aborted signal, and AbortError rejections are already treated as lifecycle teardown.

Doc/metadata sweep

• extension/README.md, extension/PRIVACY.md — navigator.modelContext → document.modelContext; the project-structure line now correctly says content-main.ts reads navigator.modelContextTesting. Privacy policy date bumped.
• package.json — npm keyword navigator.modelContext → document.modelContext.
• docs/api.md — aborted-signal paragraph rewritten; "revisit if native changes" caveat resolved.
• examples/native-harness/src/App.tsx — the "open spec question" probe comment is settled; rejection now logs PASS, resolve logs INFO (pre-152 native).
• src/hooks/useMcpTool.ts — comment notes native 152+ returns a Promise.
• CHANGELOG.md — new Unreleased section. (0.2.0 entries left as accurate history.)

Reviewer notes

• The aborted-signal change is observable behavior of the published polyfill (no-op → rejection), so it warrants a version bump on release.
• Verified: pnpm typecheck, pnpm test (174 tests, 11 files), pnpm lint all pass.