feat: implement hardcore hearts with game mode lock

Display hardcore heart textures when a world is in hardcore mode,
matching Java Edition behavior. Hearts switch between normal/hardcore
across all states (poison, wither, flash) and all HUD resolutions.

C++ changes:
- IUIScene_HUD: check isHardcore() and call SetHardcoreMode() each tick
- UIScene_HUD: send hardcore boolean to Flash via Iggy, invalidate
  SetHealth dirty check on state change to force heart redraw
- CreateWorldMenu/LoadMenu: lock game mode to Survival when hardcore
- MinecraftServer: gate server.properties hardcore override behind
  MINECRAFT_SERVER_BUILD so offline worlds preserve their saved flag

SWF changes (via new Java tools):
- AddHardcoreBitmaps: adds 10 hardcore heart bitmaps to graphics SWFs
- AddHardcoreHearts: adds 10 new frames (15-24) to health sprite
- PatchHudABC: patches HUD ActionScript bytecode with SetHardcore
  method and frame offset logic (+14 normal/poison, +6 wither)

Also updates README changelog styling with consistent ### headings.

Minecraft.Client/Common/UI/UIScene_HUD.cpp

@@ -655,6 +655,23 @@ void UIScene_HUD::SetHorseJumpBarProgress(float progress)
 	}
 }
 
+void UIScene_HUD::SetHardcoreMode(bool bHardcore)
+{
+	IggyDataValue result;
+	IggyDataValue value[1];
+	value[0].type = IGGY_DATATYPE_boolean;
+	value[0].boolval = bHardcore;
+	IggyResult out = IggyPlayerCallMethodRS ( getMovie() , &result, IggyPlayerRootPath( getMovie() ), m_funcSetHardcore , 1 , value );
+
+	// When hardcore state changes, invalidate SetHealth's dirty check
+	// so hearts are redrawn with the correct frame set on the next tick
+	if(bHardcore != m_lastHealthHardcore)
+	{
+		m_lastHealthHardcore = bHardcore;
+		m_lastMaxHealth = -1;
+	}
+}
+
 void UIScene_HUD::SetHealthAbsorb(int healthAbsorb)
 {
 	if(m_iCurrentHealthAbsorb != healthAbsorb)