fix: compact attachment history before capacity admission

[ptterjgf]

Summary

• Fix attachment capacity admission when removable durable history is the only known pressure source.
• Allow at most one guarded automatic compaction, preserve the current turn and its attachments, invalidate and re-read the transcript snapshot, and re-admit only the same physical provider/model.
• Fail closed for unknown capacity, Ensemble routes, and post-compaction requests that remain too large, with stable actionable terminal messages.
• Add deterministic router, selector, terminal-reply, and Gateway regressions.

Fixes #1394

Validation

• uv run ruff check src tests ✅
• Focused routing/Gateway/terminal tests: 180 passed, 2 skipped ✅
• Compaction/stage/harness tests: 763 passed, 2 skipped ✅
• npm --prefix opensquilla-webui run build ✅
• uv build --wheel ✅

The full local pytest run reached 24,669 passed, 624 skipped; its remaining failures/errors are unrelated Windows environment contracts (Git Bash is unavailable and some migration fixtures are read with the system GBK codec). No changed #1394 test or source file failed in the focused suites.

Third-party origins: none.