docs: expand doc.go for channelserver and mhfpacket packages

Document the Unk field naming convention used across 300+ packet
structs so new contributors understand these are intentionally
unnamed reverse-engineered protocol fields. Expand channelserver
doc.go with handler registration workflow, repository pattern,
testing approach, and lock ordering.

network/mhfpacket/doc.go

@@ -1,4 +1,14 @@
 // Package mhfpacket defines the struct representations and binary
 // serialization for every MHF network packet (~400 message types). Each
 // packet implements the [MHFPacket] interface (Parse, Build, Opcode).
+//
+// # Unk Fields
+//
+// Fields named Unk0, Unk1, … UnkN (or simply Unk) are protocol fields
+// whose purpose has not yet been reverse-engineered. They are parsed and
+// round-tripped faithfully but their semantic meaning is unknown. As
+// fields are identified through protocol research or client
+// decompilation, they should be renamed to descriptive names. The same
+// convention applies to Unk fields in handler and repository code
+// throughout the channelserver package.
 package mhfpacket