feat(audio): 使用 Windowed Sinc 高质量重采样器替代线性插值

统一音频输出采样率为 44100Hz，使用 go-audio-resampler 库实现
Windowed Sinc + Polyphase FIR 算法（VeryHigh 28-bit 精度），
替代原有的线性插值透传方案。

主要变更：
- 新增 sincResampler：三阶段 Read 循环（填充→处理→Flush）
- 双缓冲区架构避免输出样本丢失，复用内存减少 GC 压力
- WAV/MP3/BGM 播放管线全部接入 Sinc 重采样器
- 移除旧的 linearResampler 和透传模式

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/audio/sinc_resampler_test.go

@@ -0,0 +1,216 @@
+package audio
+
+import (
+	"bytes"
+	"io"
+	"math"
+	"testing"
+)
+
+// TestSincResamplerUpsampling 测试上采样 16000Hz → 44100Hz
+func TestSincResamplerUpsampling(t *testing.T) {
+	// VeryHigh 质量 FIR 延迟约 969 输入样本，数据量需远超延迟
+	inputSamples := make([]int16, 8000)
+	for i := range inputSamples {
+		inputSamples[i] = int16(math.Sin(2*math.Pi*440.0*float64(i)/16000.0) * 8000)
+	}
+
+	inputData := encodeInt16LE(inputSamples)
+	r := newSincResampler(inputData, 16000, 44100, 2).(*sincResampler)
+
+	outputSamples := readAllSamples(t, r)
+	expectedSamples := int(float64(len(inputSamples)) * 44100.0 / 16000.0)
+
+	t.Logf("输入: %d 样本 @ 16000Hz", len(inputSamples))
+	t.Logf("输出: %d 样本 @ 44100Hz (期望 ~%d)", outputSamples, expectedSamples)
+
+	if outputSamples == 0 {
+		t.Fatal("没有输出数据")
+	}
+	// 上采样：输出应多于输入
+	if outputSamples <= len(inputSamples) {
+		t.Errorf("上采样失败：输出(%d) 应多于输入(%d)", outputSamples, len(inputSamples))
+	}
+	assertWithinTolerance(t, outputSamples, expectedSamples, 0.15)
+}
+
+// TestSincResamplerPassthrough 测试采样率相同时直接透传
+func TestSincResamplerPassthrough(t *testing.T) {
+	inputSamples := []int16{100, 200, 300, 400, 500, 600}
+	inputData := encodeInt16LE(inputSamples)
+
+	r := newSincResampler(inputData, 16000, 16000, 2)
+	if _, ok := r.(*bytes.Buffer); !ok {
+		t.Error("采样率相同时应该直接透传原始 reader")
+	}
+}
+
+// TestSincResamplerDownsampling 测试下采样 44100Hz → 16000Hz
+func TestSincResamplerDownsampling(t *testing.T) {
+	inputSamples := make([]int16, 8000)
+	for i := range inputSamples {
+		inputSamples[i] = int16(math.Sin(2*math.Pi*440.0*float64(i)/44100.0) * 8000)
+	}
+
+	inputData := encodeInt16LE(inputSamples)
+	r := newSincResampler(inputData, 44100, 16000, 2).(*sincResampler)
+
+	outputSamples := readAllSamples(t, r)
+	expectedSamples := int(float64(len(inputSamples)) * 16000.0 / 44100.0)
+
+	t.Logf("输入: %d 样本 @ 44100Hz", len(inputSamples))
+	t.Logf("输出: %d 样本 @ 16000Hz (期望 ~%d)", outputSamples, expectedSamples)
+
+	if outputSamples == 0 {
+		t.Fatal("没有输出数据")
+	}
+	// 下采样：输出应少于输入
+	if outputSamples >= len(inputSamples) {
+		t.Errorf("下采样失败：输出(%d) 应少于输入(%d)", outputSamples, len(inputSamples))
+	}
+	assertWithinTolerance(t, outputSamples, expectedSamples, 0.15)
+}
+
+// TestSincResamplerFlush 测试小数据量时 Flush 获取尾部残留
+func TestSincResamplerFlush(t *testing.T) {
+	// 小数据集：输入少于 FIR 延迟，输出主要来自 Flush
+	inputSamples := make([]int16, 500)
+	for i := range inputSamples {
+		inputSamples[i] = int16(i * 100)
+	}
+
+	inputData := encodeInt16LE(inputSamples)
+	r := newSincResampler(inputData, 16000, 44100, 2).(*sincResampler)
+
+	outputSamples := readAllSamples(t, r)
+	t.Logf("小数据输入: %d 样本, 输出: %d 样本 (来自 Flush)", len(inputSamples), outputSamples)
+
+	// 即使输入小于延迟，Flush 也应产出数据
+	if outputSamples == 0 {
+		t.Fatal("Flush 未产生任何数据")
+	}
+}
+
+// TestSincResamplerShortBuffer 测试 io.Reader 边界行为
+func TestSincResamplerShortBuffer(t *testing.T) {
+	inputSamples := make([]int16, 2000)
+	for i := range inputSamples {
+		inputSamples[i] = int16(i)
+	}
+
+	inputData := encodeInt16LE(inputSamples)
+	r := newSincResampler(inputData, 16000, 44100, 2).(*sincResampler)
+
+	// 1 字节 buffer → ErrShortBuffer
+	_, err := r.Read(make([]byte, 1))
+	if err != io.ErrShortBuffer {
+		t.Errorf("期望 io.ErrShortBuffer，得到: %v", err)
+	}
+
+	// 2 字节 buffer → 正常工作
+	buf := make([]byte, 2)
+	n, err := r.Read(buf)
+	if n != 2 || err != nil {
+		t.Errorf("2 字节 buffer 应正常读取: n=%d, err=%v", n, err)
+	}
+}
+
+// TestSincResamplerStreaming 测试流式多次 Read 的正确性
+func TestSincResamplerStreaming(t *testing.T) {
+	inputSamples := make([]int16, 10000)
+	for i := range inputSamples {
+		inputSamples[i] = int16(math.Sin(2*math.Pi*440.0*float64(i)/16000.0) * 8000)
+	}
+
+	inputData := encodeInt16LE(inputSamples)
+	r := newSincResampler(inputData, 16000, 44100, 2).(*sincResampler)
+
+	// 小 buffer 模拟流式读取
+	buf := make([]byte, 128)
+	totalSamples := 0
+	readCount := 0
+
+	for {
+		n, err := r.Read(buf)
+		if n > 0 {
+			totalSamples += n / 2
+			readCount++
+		}
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("读取失败: %v", err)
+		}
+	}
+
+	expectedSamples := int(float64(len(inputSamples)) * 44100.0 / 16000.0)
+	t.Logf("流式读取: %d 次, 共 %d 样本 (期望 ~%d)", readCount, totalSamples, expectedSamples)
+
+	if readCount < 50 {
+		t.Errorf("流式读取次数过少: %d", readCount)
+	}
+	assertWithinTolerance(t, totalSamples, expectedSamples, 0.15)
+}
+
+// TestSincResamplerSineWave 测试已知正弦波信号的重采样
+func TestSincResamplerSineWave(t *testing.T) {
+	const freq = 440.0
+	const inRate = 16000
+	inputSamples := make([]int16, inRate/4) // 0.25 秒
+	for i := range inputSamples {
+		inputSamples[i] = int16(math.Sin(2*math.Pi*freq*float64(i)/float64(inRate)) * 16000)
+	}
+
+	inputData := encodeInt16LE(inputSamples)
+	r := newSincResampler(inputData, inRate, 44100, 2).(*sincResampler)
+
+	output := readAllSamples(t, r)
+	expected := int(float64(len(inputSamples)) * 44100.0 / float64(inRate))
+
+	t.Logf("440Hz 正弦波: %d → %d 样本 (期望 ~%d)", len(inputSamples), output, expected)
+
+	if output == 0 {
+		t.Fatal("正弦波重采样无输出")
+	}
+	assertWithinTolerance(t, output, expected, 0.15)
+}
+
+// --- 辅助函数 ---
+
+func encodeInt16LE(samples []int16) *bytes.Buffer {
+	buf := bytes.NewBuffer(nil)
+	for _, s := range samples {
+		buf.Write([]byte{byte(s), byte(s >> 8)})
+	}
+	return buf
+}
+
+func readAllSamples(t *testing.T, r io.Reader) int {
+	t.Helper()
+	outputData := bytes.NewBuffer(nil)
+	buf := make([]byte, 4096)
+	for {
+		n, err := r.Read(buf)
+		if n > 0 {
+			outputData.Write(buf[:n])
+		}
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("读取失败: %v", err)
+		}
+	}
+	return outputData.Len() / 2
+}
+
+func assertWithinTolerance(t *testing.T, actual, expected int, tolerance float64) {
+	t.Helper()
+	delta := math.Abs(float64(actual - expected))
+	maxDelta := float64(expected) * tolerance
+	if delta > maxDelta && delta > 10 {
+		t.Errorf("超出容忍度: 实际 %d, 期望 %d (差: %.0f, 上限: %.0f)",
+			actual, expected, delta, maxDelta)
+	}
+}