feat: complete M5 batch performance gates

[specter119]

Summary

• Reduce repeated manifest validation and journal work while preserving interruption, identity, and durability boundaries.
• Add reproducible batch benchmarks, fault-injection tests, and guard ablation probes.
• Record M5 closure evidence and update the project status and roadmap.

Verification

• Existing CI passed for head ed2525f5203476e49625f483a87400a8c39eee7c: Python 3.12, 3.13, and 3.14 test/build jobs plus wheel and sdist package smoke tests.
• CI run: https://github.com/specter119/luwu/actions/runs/35747924932
• Detailed performance, interruption, and release-gate evidence: docs/milestones/m5.md.

This PR merges the completed M5 branch into master.