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/.

network/mhfpacket/msg_mhf_update_guild_icon.go

@@ -8,6 +8,7 @@ import (
 	"erupe-ce/network/clientctx"
 )
 
+// GuildIconMsgPart represents one graphical part of a guild icon (emblem).
 type GuildIconMsgPart struct {
 	Index    uint16
 	ID       uint16