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_cb2.go

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+package amrnb
+
+// 2-pulse algebraic codebook search (9 bits, MR475/MR515), ported from
+// opencore-amrnb c2_9pf.cpp. code2i40_9bits sharpens the impulse response with
+// the pitch gain, builds the correlation gradient (corHX) and the rr criterion
+// matrix (corH), exhaustively searches the two pulse positions maximizing
+// ps²/energy (search2i40), then assembles the innovation vector, its filtered
+// version, and the codebook index (build2Code). The exact encoder counterpart
+// of the decoder's decode2i40_9bits. Bit-exact with the reference.
+
+// per-subframe track table used to encode the pulse track-pair (c2_9pf.cpp).
+var trackTable = [20]int16{
+	0, 1, 0, 1, -1, // subframe 1
+	0, -1, 1, 0, 1, // subframe 2
+	0, 1, 0, -1, 1, // subframe 3
+	0, 1, -1, 0, 1, // subframe 4
+}
+
+// search2i40 finds the two pulse positions (one per track of a track pair) that
+// maximize the codebook criterion ps²/energy. Mirrors search_2i40.
+func search2i40(subNr int16, dn, rr, codvec []int16) {
+	const L = cL_CODE
+	var ipos [2]int16
+	psk := int16(-1)
+	alpk := int16(1)
+	codvec[0] = 0
+	codvec[1] = 1
+
+	for track1 := int16(0); track1 < 2; track1++ {
+		i := (subNr << 1) + (track1 << 3)
+		ipos[0] = startPos[i]
+		ipos[1] = startPos[i+1]
+
+		for i0 := ipos[0]; i0 < L; i0 += cSTEP {
+			ps0 := dn[i0]
+			alp0 := int32(rr[int(i0)*L+int(i0)]) << 14
+
+			sq := int16(-1)
+			alp := int16(1)
+			ix := ipos[1]
+
+			for i1 := ipos[1]; i1 < L; i1 += cSTEP {
+				ps1 := ps0 + dn[i1]
+				alp1 := alp0 + (int32(rr[int(i1)*L+int(i1)]) << 14)
+				alp1 += int32(rr[int(i0)*L+int(i1)]) << 15
+				sq1 := int16((int32(ps1) * int32(ps1)) >> 15)
+				alp16 := int16((alp1 + 0x8000) >> 16)
+				s := (int32(alp)*int32(sq1))<<1 - (int32(sq)*int32(alp16))<<1
+				if s > 0 {
+					sq = sq1
+					alp = alp16
+					ix = i1
+				}
+			}
+
+			s := (int32(alpk)*int32(sq))<<1 - (int32(psk)*int32(alp))<<1
+			if s > 0 {
+				psk = sq
+				alpk = alp
+				codvec[0] = i0
+				codvec[1] = ix
+			}
+		}
+	}
+}
+
+// build2Code assembles the innovation vector cod (two ±8191/∓8192 pulses), its
+// filtered version y, the pulse signs, and the codebook index. Mirrors
+// build_code (c2_9pf.cpp).
+func build2Code(subNr int16, codvec, dnSign, cod, h, y []int16) (indx, sign int16) {
+	const L = cL_CODE
+	ptBase := int(subNr) + (int(subNr) << 2) // subNr*5
+	for i := 0; i < L; i++ {
+		cod[i] = 0
+	}
+
+	var sgn [2]int16
+	rsign := int16(0)
+	for k := 0; k < 2; k++ {
+		i := codvec[k]
+		j := dnSign[i]
+		index := int16((int32(i) * 6554) >> 15) // pos/5
+		track := i - 5*index
+		first := trackTable[ptBase+int(track)]
+		if k == 0 {
+			track = 0
+			if first != 0 {
+				index += 64
+			}
+		} else {
+			track = 1
+			index <<= 3
+		}
+		if j > 0 {
+			cod[i] = 8191
+			sgn[k] = 32767
+			rsign += 1 << uint(track)
+		} else {
+			cod[i] = -8192
+			sgn[k] = -32768
+		}
+		indx += index
+	}
+	sign = rsign
+
+	c0, c1 := int(codvec[0]), int(codvec[1])
+	for i := 0; i < L; i++ {
+		var h0, h1 int16
+		if i-c0 >= 0 {
+			h0 = h[i-c0]
+		}
+		if i-c1 >= 0 {
+			h1 = h[i-c1]
+		}
+		s := L_mult(h0, sgn[0])
+		s = L_mac(s, h1, sgn[1])
+		y[i] = round_(s)
+	}
+	return indx, sign
+}
+
+// code2i40_9bits runs the full 2-pulse search for one subframe, returning the
+// codebook index and pulse signs and writing the innovation code and its
+// filtered version y. h is sharpened in place by the pitch gain. Mirrors
+// code_2i40_9bits.
+func code2i40_9bits(subNr int16, x, h []int16, T0, pitchSharp int16, code, y []int16) (index, sign int16) {
+	sharp := shl(pitchSharp, 1)
+	if T0 < cL_CODE {
+		for i := T0; i < cL_CODE; i++ {
+			h[i] = add(h[i], mult(h[i-T0], sharp))
+		}
+	}
+
+	var dn, dn2, dnSign [cL_CODE]int16
+	var rr [cL_CODE * cL_CODE]int16
+	var codvec [2]int16
+
+	corHX(h, x, dn[:], 1)
+	setSign(dn[:], dnSign[:], dn2[:], 8)
+	corH(h, dnSign[:], rr[:])
+	search2i40(subNr, dn[:], rr[:], codvec[:])
+	index, sign = build2Code(subNr, codvec[:], dnSign[:], code, h, y)
+
+	if T0 < cL_CODE {
+		for i := T0; i < cL_CODE; i++ {
+			code[i] = add(code[i], mult(code[i-T0], sharp))
+		}
+	}
+	return index, sign
+}