BACKLOG — phase `prevb`

SSOT: /srv/cc-ci/cc-ci-plan/plan-phase-prevb-previous-dynamic-base.md.

Build backlog

B1. Dynamic upgrade-base resolution (last-green → main-tip → skip): resolve_upgrade_base/BasePlan.
B2. tests/<recipe>/previous/ mechanism: discovery, VERSION marker, base-only application, head exclusion (stripped before head redeploy), version-guard + stale-flag. Unit-tested.
B3. Discourse migration: compose.ccci.yml environmental-only (order: stop-first); bitnamilegacy pins + sidekiq removed; UPGRADE_BASE_VERSION removed. No previous/ (base deploys clean).
B4. Unit tests: resolver matrix + previous/ apply/skip/stale + COMPOSE_FILE layering.
B5. Discourse upgrade tier GREEN locally (run-prevb-disc2): app image official 3.5.3 (not bitnamilegacy), no sidekiq (pruned), version 0.8.1+3.5.0→1.0.0+3.5.3, install+upgrade pass. (Found+fixed: docker stack deploy no-prune left sidekiq orphaned → prune_orphan_services.)
B6. CLAIM M1 (clean tree + STATUS WHAT/HOW/EXPECTED/WHERE/TEETH).

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-owned section — Builder does not edit below.)