feat: evaluate deterministic cross-branch corroboration

docs/v0.6-atomicity-experiment.md

@@ -149,6 +149,40 @@ Deterministic propagation rules now enforce:
 - status changes do not raise confidence on their own
 - parent resolution still requires the separate completion rule
 
+## Cross-branch corroboration
+
+The next confidence experiment adds deterministic branch interaction checks without changing the graph model.
+
+The engine now distinguishes between:
+
+- **multiple evidence**: more than one branch exists
+- **independent corroboration**: distinct resolved branches support the same parent without sharing the same evidence key
+- **duplicate evidence**: the same evidence key appears through multiple branches and must not be double-counted
+- **conflicting evidence**: branches support incompatible positions, such as `recognised correctly` vs `recognised incorrectly`
+
+Deterministic branch rules:
+
+- corroboration only counts when branches are distinct and their evidence sources differ
+- duplicate evidence groups never count as corroboration
+- conflicts cap conclusion confidence and prevent a higher confidence upgrade
+- independent branches remain interaction-neutral
+
+Additional diagnostics now expose:
+
+- `corroboratingBranchCount`
+- `conflictingBranchCount`
+- `duplicateEvidenceCount`
+- `independentBranchCount`
+- `interactionSummary`
+- `confidenceAdjustmentReason`
+
+Observed effect:
+
+- independent corroboration can raise `evidenceConfidence`
+- duplicate evidence produces no extra confidence increase
+- conflicting evidence lowers or caps `conclusionConfidence`
+- completeness rules still dominate whether a parent may become highly justified
+
 Example progression:
 
 - parent before: `unknown`, `medium`