ByteFrame previously panicked on out-of-bounds reads, which crashed
the server when parsing malformed client packets. Now sets a sticky
error (checked via Err()) and returns zero values, matching the
encoding/binary scanner pattern. The session recv loop checks Err()
after parsing to reject malformed packets gracefully.
Also replaces remaining panic("Not implemented") stubs in network
packet Build/Parse methods with proper error returns.
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/.
