router: add MatchText to NormalizedInput and NormalizeMatchText function (slice 9a)

Add MatchText field to NormalizedInput — a lossy lexical matching view
derived from ingress text: TrimSpace → NFKC → lowercase → collapse
Unicode whitespace. Does NOT fold ё→е, strip punctuation, strip wake
words, rewrite numbers, or invoke morphology.

Both ingress sites (voice + text) construct MatchText at entry. No
existing consumer reads MatchText yet — it is dark data for future
opt-in migration.

Vikunja: #725

internal/router/source.go

@@ -92,10 +92,12 @@ const (
 )
 
 // NormalizedInput — the typed ingress boundary for a turn. Text is the raw
-// utterance after STT (voice) or as typed (text). Source identifies the
-// channel. This slice performs no new linguistic normalization: text and voice
-// paths continue to converge onto the same turn path as they did before.
+// utterance after STT (voice) or as typed (text). MatchText is a lossy
+// lexical matching view derived from Text: whitespace-collapsed, NFKC-
+// normalized, lowercased. Consumers must opt into MatchText individually;
+// nothing reads it by default in this slice. Source identifies the channel.
 type NormalizedInput struct {
-	Text   string
-	Source InputSource
+	Text      string // original ingress text — byte-for-byte the value current consumers receive
+	MatchText string // derived lossy representation for case-insensitive lexical matching
+	Source    InputSource
 }