chore(memory): BDR-063 + LRN-119/120 — GSC+CrUX data layer (OAuth token store, fail-open engine contract, SDD merge-base gotcha)

.claude/memory/learnings.md

@@ -1196,3 +1196,15 @@ rules:
 - **fix applied**: commit-change got Phase 0 = the shared `gitflow-aiguillage.md` (TYPE=chore, branch on protected base, no-op on working) + report-only fallback; client-handover push gated behind explicit-GO AskUserQuestion + report-only fallback. Dry-runs proved BOTH sides of each fallback (branch-taken AND not-taken), not just the happy path.
 - **future application**: any fleet/skill conformity audit — (1) triage by "autonomous commit/push reached?", not "file written?"; (2) read every git-signal in context (prohibition vs usage); (3) test the deterministic backstop empirically before trusting it; (4) verify a referenced lib exists + its contract matches BEFORE copying it (phantom-reference guard); (5) dry-run both branches of every fallback.
 - **cousin**: [[LRN-117]] (orphaned CODE has no sequence to check), [[LRN-034]] (narrated ≠ ground truth), [[BDR-061]] (report-only agent tool-grants).
+
+## LRN-119 — Fail-open engine contract for optional external data (real-if-connected, else graceful)
+- **pattern**: `lib/seo-data/fetch.sh` = one entrypoint; every subcmd ALWAYS emits JSON on stdout, exit 0 on ok/degraded, exit 2 on bad-usage, NEVER empty stdout, NEVER prints a secret. Third-party imports (google-auth, requests) function-local (lazy) so stdlib-only paths — mock (`SEO_DATA_MOCK_DIR`), degrade (no key/no account/revoked token), offline tests — run with no venv. Missing creds → `{"status":"degraded","reason":...}` and the caller (`/seo` analyzer) falls back to anonymous PageSpeed; audit NEVER fails on absent data. Both Python `_cli` wrapped try/except: SystemExit→bad_usage JSON+reraise, Exception→degraded JSON (corrupt store never leaks stack/path). 3rd status value `error` on exit-2 only.
+- **why it matters**: an optional-data integration must be invisible when unconfigured. Fail-CLOSED (crash/empty/nonzero) breaks every audit for users who never connect GSC. Fail-open + lazy-import keeps the 49 tests network-free and makes degrade a first-class tested branch, not an afterthought.
+- **future application**: any "use real data if credentials present, else degrade" seam — put the contract in the shell entrypoint (always-JSON / exit-code discipline), lazy-import the SDK, make degrade a returned status not an exception, test degrade+mock stdlib-only, redact secrets at the boundary (list omits token, `exec 2>/dev/null` unless debug).
+- **cousin**: [[BDR-063]] (the token store this fronts), [[LRN-120]] (SDD base gotcha, same build).
+
+## LRN-120 — SDD final-review base = `git merge-base`, NOT the ledger's recorded BASE
+- **pattern**: subagent-driven-development ledger recorded `BASE: 24b47ce` — but that was IMPLEMENTATION start (after spec+plan commits), not the branch point from develop. `git merge-base develop HEAD` = `d3e644d` (real fork). Final whole-branch review diffed against recorded BASE = 881 ins / 59 del; against true merge-base = 2163 ins / 7 del — the recorded-base diff MISLEADING (netting against a divergent line → phantom deletions). Per-task reviews unaffected (each used the correct prior feature commit).
+- **why it matters**: the final review is the last gate before merge; a wrong base hides real changes or invents fake ones. The ledger BASE is a task resume-map, not a merge-delta anchor.
+- **future application**: for ANY whole-branch/final review, derive base from `git merge-base <target> HEAD`, never a stored/remembered SHA. Sanity-check: does `git log BASE..HEAD` list ONLY this branch's commits, nothing foreign? Diff-stats differ between candidate bases → recorded one is stale, trust merge-base.
+- **cousin**: [[LRN-119]] (same GSC+CrUX build); SDD skill's own "never HEAD~1" warning (same base-selection bug class).