[beinan]

Summary

The default proposal control window may be too small for full-copy checkpoints and log rewrites. Add Budget::limit_data so callers can lower the node-wide producer ceiling while retaining the total maintenance budget and charging every outstanding reservation. Zero and upward limit changes are rejected.

Keep public compatibility tests portable by using synthetic node addresses and a fixed, independently checked layout fingerprint. The admission module remains opt-in and does not yet change production ingestion or bound already accepted replication traffic.

Refs #378.

Testing

• Admission/placement regression tests cover outstanding producer and maintenance reservations across a lowered ceiling.
• Synthetic layout fingerprint independently computed with OpenSSL.
• Full repository checks passed: workspace Rust formatting, Clippy and tests; Python pytest, Ruff and Pyright. Optional-feature tests and Clippy also passed.