chore(tests): lint rule — cap test files per production module at 2 (#3738)

* chore(tests): lint rule — cap test files per production module at 2

Adds scripts/lint-test-file-count.cjs with a ratcheted allowlist
(scripts/lint-test-file-count.allowlist.json) capturing today's
30 violating clusters as a ceiling. New entries blocked at PR time;
reductions ratchet automatically.

Wires into .github/workflows/test.yml as a new step in lint-tests.

Refs #3737

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore(tests): add docs-exempt to changeset fragment

Internal CI lint rule — no user-facing docs impact.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>