test(confidence-engine): add canonical overview experiment helper

docs/current-handoff.md

@@ -296,6 +296,8 @@ import-only proof                            — prove startCase() import chain
 OpenAI reconstruction experiment seam        — START_CASE_EXPERIMENT_PROVIDER=openai (experiment-only)
 focused-deconstruction schema fix            — plumbing verified, 48/48 tests pass
 tests/experimental/report-epistemic-ab-input-control.test.js — deterministic A/B input control apparatus (34 tests across two cases, zero live calls)
+scripts/overview-experiment-helper.cjs       — canonical overview live helper (CJS runtime)
+--self-test                                  — deterministic no-network mode with inline fake provider
 ```
 
 **No other experimental apparatus is currently valid.** Obsolete setups have been removed or superseded.
@@ -390,7 +392,8 @@ RAW USER EVIDENCE
 - `synthesizeInvestigationOverview()` previously passed the full provider envelope (`{ response: { understanding, plausibleInterpretations }, providerApiPath, providerExecution }`) directly into `validateOverviewResponse()`, which expected only `{ understanding, plausibleInterpretations }`. This caused "Missing or invalid required fields" on real providers.
 - Fix now unwraps `.response` before overview validation, matching the proven pattern in `lib/graph/current-understanding-synthesis.js` (line 296: `validateSynthesisResponse(providerResult.response)`).
 - Owning regression (`tests/graph/investigation-overview-synthesis.test.js`) now mirrors real provider envelope shape (`{ response: { understanding, plausibleInterpretations }, providerApiPath }`). Regression fails against pre-fix seam and passes after fix.
-- No live proof yet. Zero live calls made in this handoff.
+- Canonical overview live helper `scripts/overview-experiment-helper.cjs` established with deterministic no-network self-test (zero live calls). Product-launch zero-Findings fixture reaches successful overview synthesis in deterministic mode. 53/53 overview tests pass.
+- **Zero live proof yet.** Zero live calls made in this handoff.
 - Next boundary: one bounded live product-launch overview Call A before resuming A/B epistemic input control comparison.
 
 ## HISTORICAL — initial-decomposition v0.61 status