chore(prevb): bootstrap phase state + settled dynamic-base/previous decisions

machine-docs/BACKLOG-prevb.md

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+# BACKLOG — phase `prevb`
+
+SSOT: `/srv/cc-ci/cc-ci-plan/plan-phase-prevb-previous-dynamic-base.md`.
+
+## Build backlog
+
+### M1 — implemented + green locally
+- [ ] B1. Dynamic upgrade-base resolution: last-green (warm canonical registry version) → fallback
+      target-branch (`main`) tip → else skip (declared reason). Replace the static
+      `previous_version(vers[-2])` default in `run_recipe_ci.upgrade_base`. Wire into `main()` deploy.
+- [ ] B2. `tests/<recipe>/previous/` mechanism: discovery, declared-target-version marker, base-only
+      application (added to base deploy's COMPOSE_FILE), head exclusion (never applied to PR head),
+      version-guard + stale-flag on mismatch.
+- [ ] B3. Discourse migration: shrink `compose.ccci.yml` to environmental-only
+      (`order: stop-first`), delete bitnamilegacy image pins + sidekiq block; remove
+      `UPGRADE_BASE_VERSION` from `tests/discourse/recipe_meta.py`. (Expect NO `previous/`.)
+- [ ] B4. Unit tests for the new surface: base resolution (last-green / main-tip / skip), `previous/`
+      match / skip / stale, environmental-vs-version overlay layering. Update `test_upgrade_base.py`
+      to the new resolver API without weakening coverage.
+- [ ] B5. Discourse upgrade tier GREEN locally: base (bitnamilegacy:3.5.0) → head; assert deployed
+      `app` image == `discourse/discourse:3.5.3` (NOT bitnamilegacy) and no `sidekiq` service post-deploy.
+- [ ] B6. CLAIM M1 (clean tree + STATUS verification block).
+
+### M2 — proven in real CI + spot-check
+- [ ] B7. discourse PR #4 `!testme` GREEN in real CI; head ran `discourse/discourse:3.5.3`, migration exercised.
+- [ ] B8. Spot-check ≥3 other upgrade-tier recipes (warm-canonical / published-predecessor / ex-`.ccci`
+      e.g. keycloak/cryptpad/ghost) still green under dynamic base. Reconcile levels/records.
+- [ ] B9. CLAIM M2 → `## DONE` after fresh Adversary PASS on M1+M2.
+
+## Adversary findings
+(Adversary-owned section — Builder does not edit below.)