docs(confidence-engine): define finding graph reasoning contract

docs/current-handoff.md

@@ -474,3 +474,51 @@ focused contribution
 ### Decision: FINDINGS-DOMAIN-MODEL-JUSTIFIED
 
 The distinction is clearly useful enough that focused→global integration should be designed around findings/provenance rather than raw turn outputs. The semantic gap between "what one answer contributed" and "what the investigation has found so far" is real and material.
+
+---
+
+## Finding ↔ SituationGraph Reasoning Contract — August 2026
+
+1. **Finding is a separate durable reasoning entity.** It is not itself a SituationGraph node.
+
+2. **Finding ↔ graph relationship is conceptually many-to-many.** One Finding may support/challenge multiple graph propositions. Multiple Findings may support/challenge one graph proposition.
+
+3. **Findings do not directly mutate the SituationGraph.** They are evidence carriers, not graph mutation agents.
+
+4. **case/update remains authoritative for:**
+   - graph node creation/update
+   - resolving/reopening unknowns
+   - graph confidence/status changes
+   - activeUnknownNodeId
+   - selectedQuestion
+   - Current Understanding
+
+5. **Trust model:** focused reasoning proposes Findings/evidence → user disposition affects standing → case/update independently evaluates → authoritative graph changes happen only through case/update.
+
+6. **User dispositions** (Agree / Not quite / Not relevant) are semantic signals that INFORM global reasoning. They do not directly bind graph truth.
+
+7. **Finding status and graph node status remain separate concepts.** One does not dictate the other.
+
+8. **Relevance and confidence remain separate dimensions.** A Finding can be high-confidence but low-relevance, or vice versa.
+
+9. **Low-relevance Findings may remain in provenance/evidence** without affecting active investigation priority.
+
+10. **Findings may be reopened** when later evidence changes their relevance or standing.
+
+11. **Current Understanding should ultimately be reconstructed** by case/update from globally eligible Findings + graph state, not directly from the latest focused result.
+
+12. **Findings may influence but do not own:** active frontier, selectedQuestion. Only case/update sets these.
+
+13. **Raw focused fields must not be blindly copied into graph mutations.** Case/update evaluates accumulated evidence through authoritative reasoning paths (reconciliation, branch interaction analysis, decision closure protection).
+
+14. **Minimum conceptual handoff object is approximately:** Finding + disposition + provenance + relevance/standing + graph anchor. API/schema design is NOT yet settled.
+
+### Unresolved implementation choices
+
+- Finding schema
+- Finding IDs
+- contribution↔Finding mapping/clustering
+- confidence representation/scoring
+- reopening triggers
+- exact focused→global API/payload shape
+- UI presentation of Finding dispositions