v2.0.0

Release v2.0.0: MDZip spec 1.2.0 on MDZip.Core 2.0.0 (#7)

- Paths are case-sensitive after NFC; case-only matches are suggested, never used.
- create always writes a complete manifest (spec.version 1.2.0, title, entryPoint, mode);
  entry point discovery follows spec 5.5; a packaged source manifest's entryPoint is honoured.
- validate reports spec error codes and symbolic links; a missing manifest is a note.
- Spec fixtures synced to v1.2.0; command-level tests for T-013/014/015/017, P-004/005/008.

Closes #7

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>