Port ECD encryption/decryption from ReFrontier (C#) and FrontierTextHandler
(Python) into common/decryption. The cipher uses a 32-bit LCG key stream with
an 8-round Feistel-like nibble transformation and CFB chaining; all six key
sets are supported, key 4 being the default for all MHF files.
On startup, loadRengokuBinary now decrypts (ECD) and decompresses (JKR) the
binary to validate pointer bounds and entry counts, then logs a structured
summary (floor counts, spawn table counts, unique monster IDs). Failures are
non-fatal — the encrypted blob is still cached and served to clients unchanged,
preserving existing behaviour. Closes#173.
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.
