feat(regression): add tests/regression/ E2E canary suite

Three canaries (@pytest.mark.canary) drive the real cold CI lifecycle:
- good-simple: custom-html-tiny @ main (435df8fc) — fast signal, expects GREEN
- good-significant: lasuite-docs @ main (290a8ad7) — multi-service, expects GREEN
- bad-false-green: custom-html @ v5-stale-docroot (71e7326a) — expects RED

Semantic teeth: beyond exit-code, each test asserts that specific named tests
ran in results.json stages (test_serving, test_serving_and_frontend, test_content_type).
If an assertion is removed, the named test disappears → regression test fails.

Includes conftest (run_recipe_ci helper + stage_has_{passing,failing}_test),
README (cadence policy, how to run, how to add), and phase state files.

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

machine-docs/BACKLOG-regression.md

@@ -2,7 +2,12 @@
 
 ## Build backlog
 
-*(Builder-owned section — read-only for Adversary)*
+- [x] Create `tests/regression/` suite (conftest + test_canaries + README)
+- [ ] Run `good-simple` canary (custom-html-tiny main) → confirm GREEN + test_serving passes
+- [ ] Run `bad-false-green` canary (custom-html v5-stale-docroot) → confirm RED + test_content_type fails
+- [ ] Run `good-significant` canary (lasuite-docs main) → confirm GREEN + test_serving_and_frontend passes
+- [ ] Open PR for operator review (DoD item 5: NOT merged)
+- [ ] Claim gate once all canary runs are GREEN/RED as expected + PR is open
 
 ## Adversary findings