Remove gcov coverage-cache test selection (--only-changes)

[sbryngelson]

Why

The gcov coverage-cache test-selection system (--only-changes / --build-coverage-cache) is being removed because it is unreliable by construction and was delivering ~zero realized benefit. Evidence from the current tree and CI logs:

• It shipped stale. coverage.py landed on master 2026-05-27, but the committed test_coverage_cache.json.gz it ships with was built 2026-03-04 (per its _meta.created) — already ~2.6 months old at merge, with a cases_hash that no longer matches.
• Nothing refreshes the committed fallback. No workflow commits a rebuilt cache back to the repo; the rebuild-cache job only uploads a 1-day artifact + actions/cache/save. So the in-repo fallback rots monotonically.
• It silently no-ops on the jobs that matter. Real CI logs show the self-hosted Phoenix job loading the cache, then git diff failed — falling back to full test suite (shallow clone → merge-base fails). Every fallback path ends in "run the full suite," which still passes — so the breakage is invisible.
• Structurally, freshness costs ~as much as it saves (the map is derived from a full gcov build + running all tests), and UUID keys (CRC32 of the test trace) shift whenever cases.py changes.

Rather than keep patching a feature whose failure mode is invisible, this removes it cleanly. A more robust approach to test-time reduction will be proposed separately.

What's removed

• toolchain/mfc/test/coverage.py, its unit tests, and the stale test_coverage_cache.json.gz
• test CLI flags: --only-changes, --build-coverage-cache, --changes-branch
• The --only-changes dispatch + --build-coverage-cache builder in test.py
• CI: the rebuild-cache job, cache download/restore/save steps, the Fetch master for coverage diff step, the dep-check step + cases_py/dep_changed outputs, the ONLY_CHANGES env, the prune_flag in common/test.sh, common/rebuild-cache.sh, and the weekly schedule cron (which existed only to refresh the cache)

What's preserved

• --gcov builds and the CodeCov coverage reporting (coverage.yml) — that's a separate feature, untouched.
• All test functionality: ./mfc.sh test runs the full suite as before.

Verification

• ruff, lint_source, formatter clean; ./mfc.sh test -l works; --only-changes now correctly rejected.
• 17 CLI schema tests + full toolchain suite pass (the only drop is the deleted coverage unit tests).
• Both edited workflow YAMLs parse; no dangling needs/job references; net −190 lines.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.31%. Comparing base (a21bfbf) to head (9c561fc).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1460   +/-   ##
=======================================
  Coverage   61.31%   61.31%           
=======================================
  Files          72       72           
  Lines       19771    19771           
  Branches     2852     2852           
=======================================
  Hits        12123    12123           
  Misses       5699     5699           
  Partials     1949     1949           

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