v0.6.3

chore: bump to 0.6.3, fix medium bugs, update README and CHANGELOG

- Fix subscription change threshold for all types (was REAL-only)
- Fix unregister_session packet (length 4→0, remove extra payload)
- Add BOOL array read-modify-write doc note
- Bump version to 0.6.3 in Cargo.toml and C# .csproj
- Update CHANGELOG.md with comprehensive 0.6.3 release notes
- Update README.md: version badges, install snippets, test counts,
  0.6.3 highlights, fix duplicate toml block

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>