feat: port GaC-hkrpg-ps to Go with SceneCastSkill camera-detach fix

- mihoyo-variant KCP in pure Go (28-byte conv+token header), UDP gateway
  with 0xFF/0x145/0x194 handshake, ms-precision KCP clock + retransmit ticker
- protobuf for 4.6.51 generated from the AstralOS StarRail.proto (4932
  messages) plus CmdID map from // CmdID: comments (2114 ids)
- sdkserver port: query_dispatch/query_gateway (Gateserver pb + base64),
  hotfix fetch, auth endpoints, SRTools freesr-data.json upload
- gameserver handlers: login flow, scene load/spawn with real monster
  entity tracking, lineup, avatar/skin, missions, tutorials (1249+855),
  bag/equip, challenge incl. StartChallenge 1787 layout + Tierce 8973 +
  peak 8948, all raw tierce replies
- SceneCastSkill camera bug fix: only real scene monsters (tracked per
  session) trigger battle; attacking props/NPCs/air replies retcode 0
  without battle_info so the camera stays locked on the character
- end-to-end smoke test simulating a client over real UDP: handshake,
  PlayerGetToken, and both SceneCastSkill paths
- reuses the Rust PS data files unchanged (freesr-data.json, persistent,
  res.json, teleports.json, challenge_data.json, mission_unlocks.json)

internal/gameserver/packet.go

@@ -0,0 +1,106 @@
+package gameserver
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
+
+const (
+	HeadMagic uint32 = 0x9D74C714
+	TailMagic uint32 = 0xD7A152C8
+)
+
+// NetPacket is the framed protocol inside the KCP stream.
+type NetPacket struct {
+	CmdType uint16
+	Head    []byte
+	Body    []byte
+}
+
+var ErrMalformedPacket = errors.New("malformed packet")
+
+func (p *NetPacket) Encode() []byte {
+	out := make([]byte, 0, 12+len(p.Head)+len(p.Body))
+	var be32 [4]byte
+	var be16 [2]byte
+
+	binary.BigEndian.PutUint32(be32[:], HeadMagic)
+	out = append(out, be32[:]...)
+	binary.BigEndian.PutUint16(be16[:], p.CmdType)
+	out = append(out, be16[:]...)
+	binary.BigEndian.PutUint16(be16[:], uint16(len(p.Head)))
+	out = append(out, be16[:]...)
+	binary.BigEndian.PutUint32(be32[:], uint32(len(p.Body)))
+	out = append(out, be32[:]...)
+	out = append(out, p.Head...)
+	out = append(out, p.Body...)
+	binary.BigEndian.PutUint32(be32[:], TailMagic)
+	out = append(out, be32[:]...)
+	return out
+}
+
+// DecodeNetPacket parses one framed packet (must be a complete packet).
+func DecodeNetPacket(data []byte) (*NetPacket, error) {
+	if len(data) < 16 {
+		return nil, fmt.Errorf("%w: too short (%d)", ErrMalformedPacket, len(data))
+	}
+	if binary.BigEndian.Uint32(data[0:4]) != HeadMagic {
+		return nil, fmt.Errorf("%w: bad head magic", ErrMalformedPacket)
+	}
+	cmdType := binary.BigEndian.Uint16(data[4:6])
+	headLen := int(binary.BigEndian.Uint16(data[6:8]))
+	bodyLen := int(binary.BigEndian.Uint32(data[8:12]))
+	if len(data) < 12+headLen+bodyLen+4 {
+		return nil, fmt.Errorf("%w: truncated", ErrMalformedPacket)
+	}
+	if binary.BigEndian.Uint32(data[12+headLen+bodyLen:]) != TailMagic {
+		return nil, fmt.Errorf("%w: bad tail magic", ErrMalformedPacket)
+	}
+	return &NetPacket{
+		CmdType: cmdType,
+		Head:    data[12 : 12+headLen],
+		Body:    data[12+headLen : 12+headLen+bodyLen],
+	}, nil
+}
+
+// NetOperation is the 20-byte handshake control packet.
+type NetOperation struct {
+	Head   uint32
+	Param1 uint32
+	Param2 uint32
+	Data   uint32
+	Tail   uint32
+}
+
+const (
+	NetOpConnectHead uint32 = 0xFF
+	NetOpConnectTail uint32 = 0xFFFFFFFF
+	NetOpAcceptHead  uint32 = 0x145
+	NetOpAcceptTail  uint32 = 0x14514545
+	NetOpDropHead    uint32 = 0x194
+	NetOpDropTail    uint32 = 0x19419494
+)
+
+func DecodeNetOperation(data []byte) (*NetOperation, error) {
+	if len(data) < 20 {
+		return nil, fmt.Errorf("%w: netop too short", ErrMalformedPacket)
+	}
+	return &NetOperation{
+		Head:   binary.BigEndian.Uint32(data[0:4]),
+		Param1: binary.BigEndian.Uint32(data[4:8]),
+		Param2: binary.BigEndian.Uint32(data[8:12]),
+		Data:   binary.BigEndian.Uint32(data[12:16]),
+		Tail:   binary.BigEndian.Uint32(data[16:20]),
+	}, nil
+}
+
+func (o *NetOperation) Encode() []byte {
+	out := make([]byte, 20)
+	binary.BigEndian.PutUint32(out[0:4], o.Head)
+	binary.BigEndian.PutUint32(out[4:8], o.Param1)
+	binary.BigEndian.PutUint32(out[8:12], o.Param2)
+	binary.BigEndian.PutUint32(out[12:16], o.Data)
+	binary.BigEndian.PutUint32(out[16:20], o.Tail)
+	return out
+}