feat(confidence-engine): establish current understanding synthesis seam

docs/current-handoff.md

@@ -2761,27 +2761,63 @@ defect has been reduced to a defined reconstruction problem.
 
 Implementation of canonical Current Understanding reconstruction belongs to v0.50.
 
-### v0.50 — NEXT BOUNDARY
+## v0.50 — STANDALONE SYNTHESIS SEAM (2026-08-30)
 
-The first bounded question for the next branch:
+### Closed boundaries
 
-> **What minimal synthesis API/helper boundary should own the dedicated `SituationGraph + eligible Findings → Current Understanding` LLM operation?**
+**Standalone synthesis seam implemented.**
 
-Do not answer that question in this task.
+- **Domain function:** `lib/graph/current-understanding-synthesis.js`
+  - `filterEligibleFindings()` — eligibility contract: null/agree eligible; not_quite/not_relevant/rejected excluded
+  - `buildSynthesisPrompt()` — full SituationGraph (nodes + edges + centralStatement) + eligible Findings
+  - `synthesizeCurrentUnderstanding()` — DI-driven provider call, narrative-only output
+  - `validateSynthesisResponse()` / `synthesisResponseSchema` — zod schema for `{ currentUnderstanding }`
 
-Subsequent v0.50 work will need to resolve:
+- **API route:** `app/api/cases/synthesis/route.js` — `POST /api/cases/synthesis`, thin parse → invoke → return pattern
 
-1. synthesis API/helper ownership
-2. deterministic input normalization / eligible-Finding selection
-3. synthesis prompt/schema contract
-4. completed-transition integration points
-5. removal/replacement of append semantics
-6. retirement of Done-for-now CU promotion
-7. reload/cache freshness strategy
-8. targeted deterministic + live behavioural verification
+- **Domain seam ownership:**
+  - ScenarioForm → WHEN synthesis occurs (NOT wired; untouched in this increment)
+  - This module → HOW canonical state becomes narrative
+  - Provider → generation via DI (default provider + test injection pattern)
 
-This is a roadmap, not permission to implement all of it at once.
-v0.50 must continue using bounded increments.
+- **Eligibility filtering inside domain seam:**
+  - `null` → eligible (accepted-by-default, provisional working interpretation)
+  - `agree` → eligible (confirmed evidence)
+  - `not_quite` → excluded until corrected proposition saved
+  - `not_relevant` → excluded (discounted from active reasoning)
+  - `evaluation: "rejected"` → excluded
+
+- **Full SituationGraph + canonical Findings input** — not just centralStatement; nodes and edges included
+
+- **narrative-only output** — `{ currentUnderstanding }` string; no graph/Finding mutation
+
+- **52 targeted tests passing:**
+  - `tests/graph/current-understanding-synthesis.test.js` — 40/40
+  - `tests/app/api/current-understanding-synthesis-route.test.js` — 12/12
+
+- **Build result:** clean production build
+
+- **No ScenarioForm trigger integration yet** — wire-in in a future increment.
+
+### Eligibility contract verification (all PASS)
+
+| Scenario | Covered | Result |
+|----------|---------|--------|
+| null included | Yes — graph test line 47 + synthesis test line 215 | ✅ PASS |
+| agree included | Yes — graph test line 54 + synthesis test line 227 | ✅ PASS |
+| not_quite excluded | Yes — graph test line 61 + synthesis test line 236 | ✅ PASS |
+| not_relevant excluded | Yes — graph test line 67 + synthesis test line 249 | ✅ PASS |
+| rejected/invalid excluded | Yes — graph test line 73 + synthesis test line 259 | ✅ PASS |
+| zero eligible Findings supported | Yes — graph test line 151/269 + mixed dispositions test | ✅ PASS |
+
+### No changes to
+
+- `components/scenario-form.jsx`
+- `components/reasoning-workspace.jsx`
+- `app/api/cases/update/route.js`
+- Done-for-now code
+- storage
+- focused workspace
 
 ---