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_compress_test.go

@@ -0,0 +1,78 @@
+package decryption
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+)
+
+func TestPackSimpleRoundTrip(t *testing.T) {
+	tests := []struct {
+		name string
+		data []byte
+	}{
+		{"single byte", []byte{0x42}},
+		{"ascii text", []byte("hello world")},
+		{"repeated pattern", bytes.Repeat([]byte{0xAB, 0xCD}, 100)},
+		{"all zeros", make([]byte, 256)},
+		{"all 0xFF", bytes.Repeat([]byte{0xFF}, 128)},
+		{"sequential bytes", func() []byte {
+			b := make([]byte, 256)
+			for i := range b {
+				b[i] = byte(i)
+			}
+			return b
+		}()},
+		{"long repeating run", bytes.Repeat([]byte("ABCDEFGH"), 50)},
+		{"mixed", []byte{0x00, 0x01, 0x02, 0xFF, 0xFE, 0xFD, 0x80, 0x81}},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			compressed := PackSimple(tc.data)
+			got := UnpackSimple(compressed)
+			if !bytes.Equal(got, tc.data) {
+				t.Errorf("round-trip mismatch\n  want len=%d\n   got len=%d", len(tc.data), len(got))
+			}
+		})
+	}
+}
+
+func TestPackSimpleHeader(t *testing.T) {
+	data := []byte("test data")
+	compressed := PackSimple(data)
+
+	if len(compressed) < 16 {
+		t.Fatalf("output too short: %d bytes", len(compressed))
+	}
+
+	magic := binary.LittleEndian.Uint32(compressed[0:4])
+	if magic != 0x1A524B4A {
+		t.Errorf("wrong magic: got 0x%08X, want 0x1A524B4A", magic)
+	}
+
+	jpkType := binary.LittleEndian.Uint16(compressed[6:8])
+	if jpkType != 3 {
+		t.Errorf("wrong type: got %d, want 3", jpkType)
+	}
+
+	decompSize := binary.LittleEndian.Uint32(compressed[12:16])
+	if decompSize != uint32(len(data)) {
+		t.Errorf("wrong decompressed size: got %d, want %d", decompSize, len(data))
+	}
+}
+
+func TestPackSimpleLargeRepeating(t *testing.T) {
+	// 4 KB of repeating pattern — should compress well
+	data := bytes.Repeat([]byte{0xAA, 0xBB, 0xCC, 0xDD}, 1024)
+	compressed := PackSimple(data)
+
+	if len(compressed) >= len(data) {
+		t.Logf("note: compressed (%d) not smaller than original (%d)", len(compressed), len(data))
+	}
+
+	got := UnpackSimple(compressed)
+	if !bytes.Equal(got, data) {
+		t.Errorf("round-trip failed for large repeating data")
+	}
+}