router: introduce typed ingress boundary and route producer observability

First behavior-preserving slice of the Maven redesign. Establishes
explicit ingress/routing boundaries and enough observability to refactor
later without changing current routing, action, clarification, or
execution semantics.

Types introduced:
- NormalizedInput (internal/router/source.go): Text + InputSource,
  the typed ingress boundary replacing raw string at the turn entry.
- InputSource (internal/router/source.go): channel provenance enum
  (tap:voice, tap:text). Reuses the existing turnSource distinction.
- RouteProducer (internal/router/intent.go): which cascade stage
  produced the decision (grammar, heads, llm, classifier).

Changes:
- Decision carries a Producer RouteProducer field, set at each cascade
  stage (grammar, heads, LLM, classifier).
- turnRoute carries NormalizedInput instead of bare text string.
- runTurn takes NormalizedInput instead of (text, src).
- decision.Record carries InputSource and RouteProducer for
  observability; RoutingTrace persists route_producer (migration #27).
- turnSource is now a type alias for router.InputSource.

Behavior preserved:
- Stage-0 grammars unchanged: same order, same matching, same confidence.
- Cascade fallthrough order unchanged (grammar → heads → llm → classifier).
- Clarification behavior unchanged.
- Action dispatch unchanged.
- No new linguistic normalization.

cmd/mavend/reactive_notes_test.go

@@ -126,7 +126,7 @@ func TestRunTurnExplicitNoteStoresOnlyTheBody(t *testing.T) {
 	}
 
 	const utterance = "запомни: запасной ключ лежит в синей коробке"
-	if reply := h.runTurn(ctx, utterance, sourceText); reply != "сохранила заметку." {
+	if reply := h.runTurn(ctx, router.NormalizedInput{Text: utterance, Source: sourceText}); reply != "сохранила заметку." {
 		t.Fatalf("reply = %q, want the fixed feminine acknowledgement", reply)
 	}
 	if model.calls != 0 {