docs: add doc.go files and godoc comments to all packages

Add package-level documentation (doc.go) to all 22 first-party
packages and godoc comments to ~150 previously undocumented
exported symbols across common/, network/, and server/.

server/channelserver/doc.go

@@ -0,0 +1,11 @@
+// Package channelserver implements the gameplay channel server (TCP port
+// 54001+) that handles all in-game multiplayer functionality. It manages
+// player sessions, stage (lobby/quest room) state, guild operations, item
+// management, event systems, and binary state relay between clients.
+//
+// Packet handlers are organized by game system into separate files
+// (handlers_quest.go, handlers_guild.go, etc.) and registered in
+// handlers_table.go. Each handler has the signature:
+//
+//	func(s *Session, p mhfpacket.MHFPacket)
+package channelserver