fix: warn on stale boot config

[brickfrog]

Detects loaded-vs-disk config divergence on poller ticks using ServerState.read_file, emits a rate-limited WARN with diverged fields/details, and leaves live config unchanged.

Verification: moon test --target native && moon run src/bin/choir_lint --target native (pass; 2015/2015 tests).

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon test --target native
  • exit: 0
  • head: 2f315b1
  • output tail:

Warning: Main package `choir/src/bin/choir` uses blackbox-only test inputs (`_test.mbt` files) in package directory "/mnt/data/Code/choir/.choir/worktrees/ei4y-config-divergence-warn-1781373538561-302538-0/src/bin/choir". Main packages will stop generating blackbox tests in a future release. Move public behavior into a non-main package and keep the main package as an entrypoint.
4
Total tests: 2015, passed: 2015, failed: 0.

• moon run src/bin/choir_lint --target native
  • exit: 0
  • head: 2f315b1

[brickfrog]

Choir: GitHub Actions rollup reports all checks successful on the latest head — re-check review/merge policy on GitHub (not an approval or merge signal by itself).

[brickfrog]

Superseded by a fresh leaf re-landing this change off main with the 3 audit findings fixed (project_dir='' inert path, lossy rate-limit signature, decorative drift test). Original leaf was killed by the idle-watchdog while parked; continuing its branch would stack PRs. Diff preserved here for reference.