feat(scenario): add JSON scenario support and JKR type-3 compressor

Closes #172. Scenario files in bin/scenarios/ can now be authored as
.json instead of .bin — the server compiles them to wire format on
load, falling back to .bin if no .json is present.

- Add ParseScenarioBinary / CompileScenarioJSON in scenario_json.go;
  supports sub-header format (strings as UTF-8, metadata as base64),
  inline format, and raw JKR blobs.
- Add PackSimple JKR type-3 (LZ77) compressor in jpk_compress.go,
  ported from ReFrontier JPKEncodeLz.cs; round-trip tested against
  UnpackSimple.
- Fix off-by-one in processDecode (jpk.go): last literal byte was
  silently dropped for data that does not end on a back-reference.
- Wire loadScenarioBinary into handleMsgSysGetFile replacing the
  inline os.ReadFile call; mirrors the existing loadQuestBinary pattern.
- Rewrite docs/scenario-format.md with full container/sub-header spec
  and JSON schema examples.

common/decryption/jpk.go

@@ -54,7 +54,7 @@ func ProcessDecode(data *byteframe.ByteFrame, outBuffer []byte) {
 func (s *jpkState) processDecode(data *byteframe.ByteFrame, outBuffer []byte) {
 	outIndex := 0
 
-	for int(data.Index()) < len(data.Data()) && outIndex < len(outBuffer)-1 {
+	for int(data.Index()) < len(data.Data()) && outIndex < len(outBuffer) {
 		if s.bitShift(data) == 0 {
 			outBuffer[outIndex] = ReadByte(data)
 			outIndex++