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/mode.go

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+// Package amrnb implements the AMR-NB (3GPP TS 26.071 / 26.090) narrowband
+// speech codec in pure Go, with RFC 4867 RTP payload framing.
+//
+// AMR-NB operates on 8 kHz mono audio in 20 ms frames (160 samples). It defines
+// eight speech coding modes (0..7) spanning 4.75 to 12.2 kbit/s, plus a comfort
+// noise (SID) frame used by DTX.
+package amrnb
+
+// SampleRate is the fixed AMR-NB sampling rate in Hz.
+const SampleRate = 8000
+
+// FrameSamples is the number of PCM samples in one 20 ms AMR-NB frame.
+const FrameSamples = 160
+
+// Mode identifies an AMR-NB speech coding mode (0..7).
+type Mode int
+
+const (
+	Mode0475 Mode = iota // 4.75 kbit/s
+	Mode0515             // 5.15 kbit/s
+	Mode0590             // 5.90 kbit/s
+	Mode0670             // 6.70 kbit/s
+	Mode0740             // 7.40 kbit/s
+	Mode0795             // 7.95 kbit/s
+	Mode1020             // 10.2 kbit/s
+	Mode1220             // 12.2 kbit/s
+	numSpeechModes
+)
+
+// Frame type (FT) values carried in the RFC 4867 ToC byte. Values 0..7 are the
+// speech modes; the remainder describe comfort noise and no-data frames. Note
+// that AMR-NB places SID at FT 8 (AMR-WB uses FT 9).
+const (
+	FTMode0475   = 0
+	FTMode0515   = 1
+	FTMode0590   = 2
+	FTMode0670   = 3
+	FTMode0740   = 4
+	FTMode0795   = 5
+	FTMode1020   = 6
+	FTMode1220   = 7
+	FTSID        = 8  // comfort noise (SID)
+	FTSpeechLost = 14 // speech lost (RFC 4867)
+	FTNoData     = 15 // no data
+)
+
+// modeBits is the number of speech bits per AMR-NB mode (0..7). These are the
+// canonical bit counts from 3GPP TS 26.101 (and RFC 4867 Table 1a).
+var modeBits = [numSpeechModes]int{
+	95,  // 4.75
+	103, // 5.15
+	118, // 5.90
+	134, // 6.70
+	148, // 7.40
+	159, // 7.95
+	204, // 10.2
+	244, // 12.2
+}
+
+// sidBits is the number of bits in an AMR-NB SID (comfort noise) frame.
+const sidBits = 39
+
+// frameBitsByFT returns the number of payload bits for a given frame type and
+// whether the frame type is a recognized one that carries bits. SID and speech
+// modes carry bits; SPEECH_LOST and NO_DATA carry none.
+func frameBitsByFT(ft int) (bits int, ok bool) {
+	switch {
+	case ft >= FTMode0475 && ft <= FTMode1220:
+		return modeBits[ft], true
+	case ft == FTSID:
+		return sidBits, true
+	case ft == FTSpeechLost, ft == FTNoData:
+		return 0, true
+	default:
+		return 0, false
+	}
+}
+
+// frameBytesByFT returns the number of octet-aligned bytes occupied by a frame
+// of the given type (bits rounded up to whole octets).
+func frameBytesByFT(ft int) (bytes int, ok bool) {
+	bits, ok := frameBitsByFT(ft)
+	if !ok {
+		return 0, false
+	}
+	return (bits + 7) / 8, true
+}
+
+// Bits returns the number of speech bits produced by this mode.
+func (m Mode) Bits() int {
+	if m < 0 || m >= numSpeechModes {
+		return 0
+	}
+	return modeBits[m]
+}
+
+// Bytes returns the octet-aligned byte count of a frame in this mode.
+func (m Mode) Bytes() int { return (m.Bits() + 7) / 8 }
+
+// FrameType returns the RFC 4867 frame-type value for this mode.
+func (m Mode) FrameType() int { return int(m) }
+
+// Valid reports whether m is a defined speech mode.
+func (m Mode) Valid() bool { return m >= Mode0475 && m < numSpeechModes }
+
+// Bitrate returns the nominal bit rate of the mode in bit/s.
+func (m Mode) Bitrate() int {
+	switch m {
+	case Mode0475:
+		return 4750
+	case Mode0515:
+		return 5150
+	case Mode0590:
+		return 5900
+	case Mode0670:
+		return 6700
+	case Mode0740:
+		return 7400
+	case Mode0795:
+		return 7950
+	case Mode1020:
+		return 10200
+	case Mode1220:
+		return 12200
+	default:
+		return 0
+	}
+}