feat(postfx): debug HUD mostra PostFX, overrides persistents al ciclar, --postfx sense valor

- HUD (F1) afegeix línia PostFX: OFF o PostFX: <preset> [V:x.xx C:x.xx S:x.xx]
- applyPostFXPreset reaaplica overrides de CLI per preservar-los en ciclar amb X
- setPostFXParamOverrides guarda els valors en membres privats per persistència
- --postfx sense valor ja no dona error i utilitza complet (preset 3) per defecte

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

source/ui/ui_manager.cpp

@@ -335,6 +335,23 @@ void UIManager::renderDebugHUD(const Engine* engine,
     lines.push_back(logic_res_text);
     lines.push_back(refresh_text);
     lines.push_back(theme_text);
+    std::string postfx_text;
+    if (!engine->isPostFXEnabled()) {
+        postfx_text = "PostFX: OFF";
+    } else {
+        static constexpr const char* preset_names[4] = {
+            "Vinyeta", "Scanlines", "Cromatica", "Complet"
+        };
+        int mode = engine->getPostFXMode();
+        char buf[64];
+        SDL_snprintf(buf, sizeof(buf), "PostFX: %s [V:%.2f C:%.2f S:%.2f]",
+                     preset_names[mode],
+                     engine->getPostFXVignette(),
+                     engine->getPostFXChroma(),
+                     engine->getPostFXScanline());
+        postfx_text = buf;
+    }
+    lines.push_back(postfx_text);
     lines.push_back(elapsed_text);
 
     const Ball* first_ball = scene_manager->getFirstBall();