plan: queue samever (older-base fallback when last-green==head, opus); IDEAS: defer canonical-history (B)

Operator 2026-06-17. Closes the prevb resolver gap: when the last-green
warm-canonical base version equals the PR head version, step back to the
newest published version strictly older than head (design A) instead of a
same-version no-op or a skip. Design B (canonical history for a green-verified
older base) saved to IDEAS. Auto-runs after regall (watchdog advances + switches
to opus).

cc-ci-plan/IDEAS.md

@@ -189,3 +189,22 @@ item into the project `BACKLOG.md` as `[idea]` if/when it becomes relevant.
     harness `finally` tears down its stack).
   - *When to revisit:* if CI-queue starvation recurs (several recipes in flight, or a legitimately
     long deploy wedging others). *Added:* 2026-06-09.
+
+- [ ] **Canonical *history* for a green-verified older upgrade base (design B).** *(operator-flagged
+  2026-06-17; deferred from the `samever` fix — design A shipped instead)*
+  Context: the dynamic upgrade-base resolver (phase `prevb`) uses the last-green warm-canonical as the
+  base. When that canonical version **equals the PR head version** (a non-version-bump PR, or a re-test
+  after the canonical advanced), phase `samever` (design A) steps back to the **newest published version
+  strictly older than head** (from `recipe_tags`). That older *published* version isn't guaranteed to
+  have passed green on cc-ci.
+  **The improvement (B):** keep a short **history** in the warm-canonical registry — the last N green
+  `{version, commit}` records, not just the single current one (`canonical.read_registry`/`write_registry`
+  are single-record today). When stepping back, prefer the most-recent **prior canonical** (green-verified)
+  over a raw published tag; fall back to design A's published-tag only when no prior canonical exists.
+  - *Why deferred:* history only accrues going forward (existing recipes have none until they've gone
+    green ≥2× on cold-on-latest), so design A (published-tag) is the always-present floor and must exist
+    anyway. B is a quality refinement on top. Requires a registry schema change (single record → bounded
+    history) + `promote_canonical`/`write_registry` appending instead of overwriting.
+  - *Re-entry trigger:* when the published-tag fallback proves insufficient — e.g. a recipe's
+    newest-older-published version is itself undeployable, so we'd rather upgrade from a known-green prior
+    canonical — or operator request. *Added:* 2026-06-17.