[BUG] Update vendored FlatBuffers to v25.12.19

[oberstet]

Summary

The vendored FlatBuffers in deps/flatbuffers is currently at v25.9.23, but the latest stable release is v25.12.19.

https://github.com/google/flatbuffers/releases/tag/v25.12.19

Current State

$ git -C deps/flatbuffers describe --tags --always
v25.9.23

Required Changes

1. Bump deps/flatbuffers submodule to v25.12.19
2. Update vendored FlatBuffers Python runtime (src/zlmdb/flatbuffers/ or src/autobahn/flatbuffers/)
3. Regenerate reflection files (reflection.bfbs + Python wrappers)
4. Update version tracking files
5. Run tests to verify compatibility

Developer Workflow

# Bump submodule
just bump-flatbuffers

# Update vendored Python runtime
just update-flatbuffers

# Regenerate reflection files (once #107 fix is in place)
just build-reflection

# Test
just check

# Commit all changes
git add deps/flatbuffers src/*/flatbuffers/
git commit -m "Bump vendored FlatBuffers to v25.12.19"

Context

See discussion in:

• zlmdb#106 (PR comment)
• zlmdb#107 (cross-compilation issue)

The vendored FlatBuffers includes:

• flatc compiler (bundled in wheels)
• Python runtime library
• Reflection schema and generated wrappers

All components should be kept in sync with the same FlatBuffers version.

Checklist

• I have searched existing issues to avoid duplicates
• I have provided a minimal reproducible example
• I have included version information
• I have included error messages/logs