Package-level globals mShiftIndex and mFlag were shared across
concurrent goroutines calling UnpackSimple from quest handlers.
Replace with a local jpkState struct scoped to each decompression
call. Add concurrent safety test validated with -race.
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/.
Fix unchecked error returns on bf.Seek(), db.Exec(), QueryRow().Scan(),
pkt.Build(), logger.Sync(), and binary.Write() calls. The linter now
passes with 0 errors, build compiles, and all tests pass with -race.
