mavsttd: silence gate — don't feed non-speech to whisper

Whisper hallucinates subtitle-credit boilerplate ("Редактор субтитров …") on
silence/room-noise, which then got stored as tap:voice facts. Gate before the
model: drop clips shorter than -min-ms (default 300) or below -silence-rms
(default 0.01 normalized RMS). Both are flags — the mic floor is hardware
specific. Returns empty transcript (same as whisper's no-segments path), so
nothing downstream changes.

gateReason is pure and unit-tested (silence/short/quiet → dropped, loud+long →
passes). ponytail: energy gate, not a real VAD; upgrade to WebRTC VAD or
whisper no_speech_prob if too blunt.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/mavsttd/silence_test.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"math"
+	"testing"
+)
+
+// sine builds `ms` milliseconds of a 16 kHz tone at the given amplitude, so we
+// can exercise the energy gate without real audio or whisper.
+func sine(ms int, amp float64) []float32 {
+	n := whisperSampleRate * ms / 1000
+	out := make([]float32, n)
+	for i := range out {
+		out[i] = float32(amp * math.Sin(2*math.Pi*440*float64(i)/whisperSampleRate))
+	}
+	return out
+}
+
+func TestGateReason(t *testing.T) {
+	const (
+		minMs  = 300
+		minRMS = 0.01
+	)
+	tests := []struct {
+		name    string
+		samples []float32
+		gated   bool // true ⇒ expect a non-empty reason (dropped)
+	}{
+		{"empty", nil, true},
+		{"too short", sine(100, 0.5), true},                 // 100ms < 300ms
+		{"long but silent", make([]float32, whisperSampleRate), true}, // 1s of zeros
+		{"long but near-silent", sine(500, 0.005), true},    // rms ~0.0035 < floor
+		{"real speech-ish", sine(500, 0.3), false},          // loud enough, long enough
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			reason := gateReason(tc.samples, whisperSampleRate, minMs, minRMS)
+			if (reason != "") != tc.gated {
+				t.Fatalf("gateReason=%q, want gated=%v", reason, tc.gated)
+			}
+		})
+	}
+}