feat(capability): 转码能力包（vendor 纯 Go AMR-NB 解码器 + 黄金向量逐样本对照 + 8 哨兵 + 注册面）

internal/capability/transcode/amrnb/dsp_enc_filt.go

@@ -0,0 +1,20 @@
+package amrnb
+
+// Encoder analysis-by-synthesis filter kernels, ported from opencore-amrnb
+// convolve.cpp. convolve computes the (truncated) lower-triangular convolution
+// y[n] = sum_{i=0..n} x[i]·h[n-i] >> 12, used throughout the closed-loop pitch
+// and algebraic codebook searches to filter an excitation by the weighted
+// synthesis impulse response. The reference accumulates in non-saturating int32,
+// so a straightforward double loop is bit-exact.
+func convolve(x, h, y []int16, L int) {
+	xf := x[:L] // hoist bounds: i<=n<L and n-i in [0,n]
+	hf := h[:L]
+	for n := 0; n < L; n++ {
+		var s int32
+		hn := hf[:n+1] // h[n-i] == hn[n-i], index in [0,n]
+		for i := 0; i <= n; i++ {
+			s += int32(xf[i]) * int32(hn[n-i])
+		}
+		y[n] = int16(s >> 12)
+	}
+}