audio and tts sweep: walk WAV chunks, name the stub sample rate (V-581)

The WAV parser now walks chunk headers to find the data chunk instead of
scanning for the four bytes "data". A LIST chunk between fmt and data is
common, arecord and ffmpeg both write one, and its payload is free text that
can spell the word. A byte scan took that text for a chunk header and read the
comment as samples.

WAVHeader named WAVFromPCM in its error, so a caller of WAVHeader read the
wrong function. internal/capture calls it twice.

PCMFromWAV returns PCM that aliases the buffer it was given. That is the right
trade for a long recording and it was undocumented, so the doc comment now says
so and names the two ways a caller gets it wrong.

The TTS stub wrote 16000 three times. It reads the rate and the sample width
off audio.PCM16kMono now, so the tone stays in tune with the shape the seam
declares, and the sample write goes through binary.LittleEndian.

Identify computed the clip length twice to report it once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/tts/lexicon_test.go

@@ -23,6 +23,9 @@ func TestLexiconRewritesNames(t *testing.T) {
 		// Two names in a row share the space between them, which one pass
 		// would consume.
 		{"GPU GPU", "джи-пи-ю джи-пи-ю"},
+		// Two passes cover a run of any length, because the first pass takes
+		// every other name and leaves both boundaries of the ones it skipped.
+		{"GPU GPU GPU GPU", "джи-пи-ю джи-пи-ю джи-пи-ю джи-пи-ю"},
 		// Not a word boundary: a name inside a longer token is left alone.
 		{"vikunjaless", "vikunjaless"},
 		{"ничего не совпало", "ничего не совпало"},

internal/tts/tts.go

@@ -20,6 +20,7 @@ package tts
 
 import (
 	"context"
+	"encoding/binary"
 	"fmt"
 	"math"
 
@@ -50,17 +51,20 @@ func NewStub() *Stub { return &Stub{} }
 // silent no-op a bug could hide behind).
 func (s *Stub) Synthesize(_ context.Context, text string) (audio.Audio, error) {
 	const durMs = 200
-	const samples = 16000 * durMs / 1000 // 3200 samples @ 16k
-	pcm := make([]byte, samples*2)
+	// The rate is read off the canonical format rather than written again, so
+	// the tone stays in tune with the shape the seam declares.
+	rate := audio.PCM16kMono.SampleRate
+	bytesPerSample := audio.PCM16kMono.SampleBits / 8
+	samples := rate * durMs / 1000
+	pcm := make([]byte, samples*bytesPerSample)
 	freq := 220.0 // A3
 	if len(text) > 0 {
 		freq = 180.0 + float64(text[0]%6)*60 // 180..480 Hz band
 	}
 	for i := 0; i < samples; i++ {
-		t := float64(i) / 16000.0
+		t := float64(i) / float64(rate)
 		v := int16(12000 * math.Sin(2*math.Pi*freq*t))
-		pcm[i*2] = byte(v)
-		pcm[i*2+1] = byte(v >> 8)
+		binary.LittleEndian.PutUint16(pcm[i*2:], uint16(v))
 	}
 	return audio.Audio{Format: audio.PCM16kMono, Bytes: pcm}, nil
 }