feat(02-02): stereo mixer utilities with constant-power pan law

- PanGains uses cos/sin constant-power pan law (D-11)
- StereoFramesToInt16Bytes converts stereo frames to interleaved LE int16 bytes
- clamp prevents int16 overflow for values outside [-1.0, 1.0]
- All 7 mixer tests pass (pan law, byte conversion, clamping)

synth/mixer.go

@@ -0,0 +1,40 @@
+package synth
+
+import (
+	"encoding/binary"
+	"math"
+)
+
+// PanGains returns left and right channel gains for a pan position p in [-1, 1].
+// Uses constant-power (equal-power) pan law: cos/sin mapping.
+// Per D-11: bass frequencies center, mid spread L/R, higher frequencies wider.
+// At p=-1.0: gainL=1.0, gainR=0.0; at p=0.0: gainL=gainR=sqrt(2)/2; at p=1.0: gainL=0.0, gainR=1.0.
+func PanGains(p float64) (gainL, gainR float64) {
+	angle := (p + 1.0) / 2.0 * math.Pi / 2.0
+	return math.Cos(angle), math.Sin(angle)
+}
+
+// StereoFramesToInt16Bytes converts [][2]float64 stereo frames to interleaved
+// little-endian int16 bytes suitable for go-lame's Write method.
+// Clamps values to [-1.0, 1.0] before conversion.
+// Output format: [L0_lo, L0_hi, R0_lo, R0_hi, L1_lo, L1_hi, R1_lo, R1_hi, ...]
+func StereoFramesToInt16Bytes(frames [][2]float64) []byte {
+	buf := make([]byte, len(frames)*4) // 2 channels * 2 bytes per sample
+	for i, frame := range frames {
+		l := clamp(frame[0])
+		r := clamp(frame[1])
+		binary.LittleEndian.PutUint16(buf[i*4:], uint16(int16(l*32767)))
+		binary.LittleEndian.PutUint16(buf[i*4+2:], uint16(int16(r*32767)))
+	}
+	return buf
+}
+
+func clamp(v float64) float64 {
+	if v > 1.0 {
+		return 1.0
+	}
+	if v < -1.0 {
+		return -1.0
+	}
+	return v
+}