router/semantic: coarse-route contract, interface, and legacy mapping (slice 12)

Defines the six-class SemanticRoute type (conversation, knowledge,
action, memory_write, system, uncertain), the SemanticRouteDecision
output, the SemanticRouter interface, and the deterministic
Intent→SemanticRoute mapping from the current seven-intent cascade.

internal/router/semantic/interface.go

@@ -0,0 +1,14 @@
+package semantic
+
+import "context"
+
+// SemanticRouter — the interface behind which experiment models run. The
+// implementation decides whether to consume MatchText; the contract here is
+// that Route returns a coarse decision without touching the authoritative
+// router's Intent, slots, capability, or source.
+//
+// A nil implementation is a valid floor — the shadow harness records
+// "no model" and continues.
+type SemanticRouter interface {
+	Route(ctx context.Context, text string) (SemanticRouteDecision, error)
+}