router: ResolveActionCandidate reads CapabilitySelection first (slice 6b)

The candidate now receives Fn/Args from CapabilitySelection (the
authoritative record) rather than from Decision.Slots.HasFn. Backward
compatibility: decisions with Slots.HasFn but no CapabilitySelection
(tests, rebuilt decisions) still resolve via the compatibility path.

internal/router/actioncandidate.go

@@ -130,21 +130,37 @@ func (r ActionValidationResult) Valid() bool { return r.Status == ActionValid }
 //
 // Resolution rules:
 //   - Non-act intents: candidate is not applicable (Fn empty, source empty).
-//   - Act with Slots.HasFn: the router already resolved the function upstream
-//     (stage-0 grammar, stage-2 extractor, or LLM slot backfill). Candidate
-//     source is ActionSourceRoute.
-//   - Act without Fn: the fallback matcher runs against the text slot.
-//     Candidate source is ActionSourceMatcher on match, or Fn stays empty.
+//   - Act with resolved CapabilitySelection: the capability-selection stage
+//     already resolved the function. Candidate source is ActionSourceRoute.
+//   - Act without resolved CapabilitySelection: the fallback matcher runs
+//     against the text slot. Candidate source is ActionSourceMatcher on match,
+//     or Fn stays empty.
 //
-// The matcher algorithm, enabled-tool set, alias behavior, fuzzy-prefix
-// behavior, and ordering are unchanged — this is a mechanical extraction of
-// the same matching call that actionAct previously owned.
+// The CapabilitySelection is the authoritative source for Fn/Args. The
+// compatibility fields on Decision.Slots (Fn/Args/HasFn) are still populated
+// for backward compatibility but are not read here.
 func ResolveActionCandidate(dec Decision, m ActMatcher) ActionCandidate {
 	if dec.Intent != IntentAct {
 		return ActionCandidate{}
 	}
 
-	// Router resolved the function upstream.
+	// Capability selection resolved the function upstream. This is the
+	// authoritative path for decisions produced by the router (which calls
+	// SelectCapability).
+	if dec.CapabilitySelection.Resolved {
+		return ActionCandidate{
+			Fn:         dec.CapabilitySelection.Fn,
+			Args:       dec.CapabilitySelection.Args,
+			Source:     ActionSourceRoute,
+			Producer:   dec.Producer,
+			ResolvedBy: dec.CapabilitySelection.Method,
+			Confidence: dec.Confidence,
+		}
+	}
+
+	// Backward compatibility: decisions constructed outside the router
+	// (tests, rebuilt decisions) may set Slots.HasFn without
+	// CapabilitySelection. Read the compatibility fields.
 	if dec.Slots.HasFn {
 		return ActionCandidate{
 			Fn:         dec.Slots.Fn,