feat(03-05): ajoute 4 champs Visual* optionnels + round-trip tests (Task 1)

- GravityFlipRegion: VisualColor/VisualMode/VisualRefreshMs/VisualOpacity
  avec defaults (#00FFFF, Outline, 1000, 0.5), getters/setters, javadoc.
- CODEC: 4 KeyedCodec optionnels (pas de validator nonNull) => back-compat
  legacy regions.json.
- 2 tests round-trip (custom + defaults quand champs absents).

src/test/java/com/mythlane/gravityflip/region/GravityFlipRegionCodecTest.java

@@ -144,6 +144,41 @@ class GravityFlipRegionCodecTest {
         assertFalse(decoded.isAffectItems());
     }
 
+    // ---------- Plan 03-05 : 4 visualization fields ----------
+
+    @Test
+    void roundTripPreservesVisualFields() {
+        GravityFlipRegion src = baseRegion();
+        src.setVisualColor("#FF8800");
+        src.setVisualMode("Faces");
+        src.setVisualRefreshMs(500);
+        src.setVisualOpacity(0.75);
+
+        GravityFlipRegion decoded = roundTrip(src);
+
+        assertEquals("#FF8800", decoded.getVisualColor());
+        assertEquals("Faces", decoded.getVisualMode());
+        assertEquals(500, decoded.getVisualRefreshMs());
+        assertEquals(0.75, decoded.getVisualOpacity(), 1e-9);
+    }
+
+    @Test
+    void roundTripPreservesVisualDefaultsWhenFieldsAbsent() {
+        // Region construite via constructeur legacy 3-arg — simule un regions.json legacy
+        // (ni 03-04 ni 03-05 présents).
+        GravityFlipRegion src = new GravityFlipRegion(
+                "legacy-viz",
+                new Box(new Vector3d(0, 0, 0), new Vector3d(1, 1, 1)),
+                true);
+
+        GravityFlipRegion decoded = roundTrip(src);
+
+        assertEquals("#00FFFF", decoded.getVisualColor(), "default VisualColor=#00FFFF (cyan)");
+        assertEquals("Outline", decoded.getVisualMode(), "default VisualMode=Outline");
+        assertEquals(1000, decoded.getVisualRefreshMs(), "default VisualRefreshMs=1000");
+        assertEquals(0.5, decoded.getVisualOpacity(), 1e-9, "default VisualOpacity=0.5");
+    }
+
     private static GravityFlipRegion baseRegion() {
         return new GravityFlipRegion(
                 "r",