feat(channelserver): decouple channel servers for independent operation (#33)

Enable multiple Erupe instances to share a single PostgreSQL database
without destroying each other's state, fix existing data races in
cross-channel access, and lay groundwork for future distributed
channel server deployments.

Phase 1 — DB safety:
- Scope DELETE FROM servers/sign_sessions to this instance's server IDs
- Fix ci++ bug where failed channel start shifted subsequent IDs

Phase 2 — Fix data races in cross-channel access:
- Lock sessions map in FindSessionByCharID and DisconnectUser
- Lock stagesLock in handleMsgSysLockGlobalSema
- Snapshot sessions/stages under lock in TransitMessage types 1-4
- Lock channel when finding mail notification targets

Phase 3 — ChannelRegistry interface:
- Define ChannelRegistry interface with 7 cross-channel operations
- Implement LocalChannelRegistry with proper locking
- Add SessionSnapshot/StageSnapshot immutable copy types
- Delegate WorldcastMHF, FindSessionByCharID, DisconnectUser to Registry
- Migrate LockGlobalSema and guild mail handlers to use Registry
- Add comprehensive tests including concurrent access

Phase 4 — Per-channel enable/disable:
- Add Enabled *bool to EntranceChannelInfo (nil defaults to true)
- Skip disabled channels in startup loop, preserving ID stability
- Add IsEnabled() helper with backward-compatible default
- Update config.example.json with Enabled field

server/channelserver/channel_registry.go

@@ -0,0 +1,58 @@
+package channelserver
+
+import (
+	"erupe-ce/network/mhfpacket"
+	"net"
+)
+
+// ChannelRegistry abstracts cross-channel operations behind an interface.
+// The default LocalChannelRegistry wraps the in-process []*Server slice.
+// Future implementations may use DB/Redis/NATS for multi-process deployments.
+type ChannelRegistry interface {
+	// Worldcast broadcasts a packet to all sessions across all channels.
+	Worldcast(pkt mhfpacket.MHFPacket, ignoredSession *Session, ignoredChannel *Server)
+
+	// FindSessionByCharID looks up a session by character ID across all channels.
+	FindSessionByCharID(charID uint32) *Session
+
+	// DisconnectUser disconnects all sessions belonging to the given character IDs.
+	DisconnectUser(cids []uint32)
+
+	// FindChannelForStage searches all channels for a stage whose ID has the
+	// given suffix and returns the owning channel's GlobalID, or "" if not found.
+	FindChannelForStage(stageSuffix string) string
+
+	// SearchSessions searches sessions across all channels using a predicate,
+	// returning up to max snapshot results.
+	SearchSessions(predicate func(SessionSnapshot) bool, max int) []SessionSnapshot
+
+	// SearchStages searches stages across all channels with a prefix filter,
+	// returning up to max snapshot results.
+	SearchStages(stagePrefix string, max int) []StageSnapshot
+
+	// NotifyMailToCharID finds the session for charID and sends a mail notification.
+	NotifyMailToCharID(charID uint32, sender *Session, mail *Mail)
+}
+
+// SessionSnapshot is an immutable copy of session data taken under lock.
+type SessionSnapshot struct {
+	CharID       uint32
+	Name         string
+	StageID      string
+	ServerIP     net.IP
+	ServerPort   uint16
+	UserBinary3  []byte // Copy of userBinaryParts index 3
+}
+
+// StageSnapshot is an immutable copy of stage data taken under lock.
+type StageSnapshot struct {
+	ServerIP     net.IP
+	ServerPort   uint16
+	StageID      string
+	ClientCount  int
+	Reserved     int
+	MaxPlayers   uint16
+	RawBinData0  []byte
+	RawBinData1  []byte
+	RawBinData3  []byte
+}