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.

internal/decision/decision.go

@@ -68,6 +68,16 @@ type Record struct {
 	Winner    string    `json:"winner"`
 	Claims    []Claim   `json:"claims"`
 
+	// InputSource — which channel the utterance arrived on (tap:voice or
+	// tap:text). Carried for observability so a trace can distinguish a voice
+	// turn from a text turn without re-deriving it from surrounding claims.
+	InputSource string `json:"input_source,omitempty"`
+
+	// RouteProducer — which cascade stage produced the routing decision.
+	// Carried for observability so a trace names the winning component directly
+	// rather than requiring a scan of the claims list.
+	RouteProducer string `json:"route_producer,omitempty"`
+
 	mu      sync.Mutex
 	rosters []roster
 }
@@ -152,9 +162,10 @@ func (r *Record) Finish(now time.Time) *Record {
 type recorderKey struct{}
 
 // With returns a context carrying a fresh record, and the record to read after
-// the turn has answered.
-func With(ctx context.Context, utterance string) (context.Context, *Record) {
-	rec := &Record{Utterance: utterance}
+// the turn has answered. inputSource identifies the channel the utterance
+// arrived on; pass empty if unknown.
+func With(ctx context.Context, utterance string, inputSource string) (context.Context, *Record) {
+	rec := &Record{Utterance: utterance, InputSource: inputSource}
 	return context.WithValue(ctx, recorderKey{}, rec), rec
 }