router: add CapabilitySelection field to Decision (slice 6b)

Authoritative record of which executable capability matched, separate
from Decision.Intent (what kind of turn) and ActionCandidate (downstream
action artifact). Decision.Slots.Fn/Args/HasFn remain as compatibility
representations populated from this selection.

internal/router/intent.go

@@ -150,6 +150,16 @@ type Decision struct {
 	Slots      Slots
 	Clarify    bool // stage 3: below threshold — ask, don't guess
 
+	// CapabilitySelection — the authoritative result of the capability-
+	// selection stage. Says what executable capability matched, separate
+	// from what kind of turn this is (Decision.Intent) and separate from
+	// the downstream action artifact (ActionCandidate).
+	//
+	// Decision.Slots.Fn/Args/HasFn remain as compatibility representations
+	// populated FROM this selection. Later action execution must read the
+	// candidate produced from this selection, not the compatibility fields.
+	CapabilitySelection CapabilitySelection
+
 	// Producer — which cascade stage produced this decision. Recorded for
 	// observability so a trace can name the winning component directly.
 	Producer RouteProducer