[beinan]

Summary

Multiple WAL groups on one node need a shared proposal budget so concurrent producers cannot each consume the full disk allowance. Add an opt-in admission module with atomic reservations, drop-based release, encoded-write accounting, and a reserved control window. Refresh uses real file lengths and allocated blocks; successful admission does not scan the directory.

This module is not connected to production ingestion. It does not reserve filesystem extents or cap previously accepted replication traffic; replica-pressure coordination and snapshot collection remain necessary.

Testing

• Release admission/placement tests: five passed, including racing reservations and reclamation while proposals remain pending.
• Release Clippy with lab,placement,admission, formatting, spelling and diff checks passed.
• Full local CI script passed: workspace Rust Clippy/tests, Python pytest, Ruff format/lint and Pyright.
• CI explicitly tests and lints the optional module.

Refs #378.