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_clltp.go

@@ -0,0 +1,53 @@
+package amrnb
+
+// Closed-loop long-term-prediction (adaptive codebook) step, ported from
+// opencore-amrnb cl_ltp.cpp. clLtp runs the fractional pitch search (pitchFr),
+// builds the adaptive codebook (predLt), filters it (convolve), computes the
+// pitch gain (gPitch) with mode-specific clamping (and MR122 quantization), then
+// forms the codebook-search target xn2 = xn - gain·y1 and updates res2. The
+// tone-stabilization gain clip (gpc_flag) is not applied (active-speech path).
+// Bit-exact with the reference for that path.
+
+const cGP_CLIP = 15565 // 0.95 Q14 pitch-gain clip
+
+type clLtpResult struct {
+	T0, T0frac      int16
+	gainPit         int16
+	gpLimit         int16
+	gCoeff          [5]int16
+	pitchIndex      int16
+	gainPitIndex    int16 // MR122 only
+	hasGainPitIndex bool
+}
+
+// clLtp performs the adaptive-codebook analysis for one subframe. exc[excOff] is
+// the current subframe; predLt writes it in place. xn2 (codebook target) and
+// res2 (residual) are written. Mirrors cl_ltp.
+func (st *pitchFrState) clLtp(mode, frameOffset int, Top, h1, exc []int16, excOff int, res2, xn, xn2, yl []int16) clLtpResult {
+	var r clLtpResult
+	resu3 := int16(0)
+	r.T0, r.T0frac, resu3, r.pitchIndex = st.pitchFr(mode, Top, exc, excOff, xn, h1, cL_SUBFR, frameOffset)
+
+	predLt(exc, excOff, r.T0, r.T0frac, cL_SUBFR, resu3)
+	convolve(exc[excOff:], h1, yl, cL_SUBFR)
+
+	r.gainPit = gPitch(mode, xn, yl, r.gCoeff[:], cL_SUBFR)
+	r.gpLimit = maxInt16 // tone-stabilization clip disabled
+
+	if mode == dMR475 || mode == dMR515 {
+		if r.gainPit > 13926 {
+			r.gainPit = 13926
+		}
+	} else if mode == dMR122 {
+		var cand, cind [3]int16
+		r.gainPitIndex = qGainPitch(dMR122, r.gpLimit, &r.gainPit, cand[:], cind[:])
+		r.hasGainPitIndex = true
+	}
+
+	g := r.gainPit
+	for i := 0; i < cL_SUBFR; i++ {
+		xn2[i] = xn[i] - int16((int32(yl[i])*int32(g))>>14)
+		res2[i] -= int16((int32(exc[excOff+i]) * int32(g)) >> 14)
+	}
+	return r
+}