fix(watcher): prune sustained missing roots

[pcristin]

What does this PR do?

Implements the watcher stale-root cleanup discussed in #286:

• Track consecutive missing project roots in the watcher.
• Preserve projects through transient missing-root checks, then prune after sustained absence.
• Delete cached project DB/WAL/SHM files when sustained absence is confirmed.
• Cover the transient-miss and sustained-cleanup behavior with a regression test.

Fixes #286.
Supersedes #666 after refreshing the branch/title/commit metadata to match the contribution guide.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects unsigned commits.
• Tests pass locally (make -f Makefile.cbm test).
• Lint passes (make -f Makefile.cbm lint-ci).
• New behavior is covered by a test (reproduce-first for bug fixes).

Validation

• bash scripts/check-no-test-skips.sh
• git diff --check
• make -f Makefile.cbm build/c/test-runner
• build/c/test-runner was run after rebasing; the watcher regression passed. The full local runner still had unrelated local/environment failures.