Define canonical Cyd Bluesky archive v2 contract and fixtures

[micahflee]

Closes #675

Summary

• define the normative Cyd Bluesky archive v2 ZIP layout, platform-qualified metadata, integrity, completeness, privacy boundary, supported-version behavior, and Bluesky version history
• add the canonical SQLite Bluesky interchange schema covering identity, profiles, every selection category, direct context, relationships, observations, deletion state, assets, chats, and portable settings
• add reproducible complete and incomplete .cyd fixtures with valid independent PNG assets, a playable FFmpeg-generated full-video asset, and explicit missing-media semantics
• publish normalized semantic expectations for identity, profiles, records, relationships, conversations, observations, deletion state, settings, asset roles, digests, completeness, and version behavior
• verify full normalized fixture meaning, valid PNG framing and CRCs, MP4 movie/sample structure, manifest integrity, completeness, platform scoping, and Desktop version behavior

Validation

• npm run lint
• npx vitest run src/bluesky_archive_v2/__tests__/canonicalFixtures.test.ts --silent (11 tests)
• npx vitest run src --silent (89 files, 1,171 tests)
• deterministic fixture regeneration and git diff --check

(This was written by an LLM.)