feat: Phase 1 - safe constructors and _Drawable foundation

Closes #7 - Make all UI class constructors safe:
- Added safe default constructors for UISprite, UIGrid, UIEntity, UICaption
- Initialize all members to predictable values
- Made Python init functions accept no arguments
- Added x,y properties to UIEntity

Closes #71 - Create _Drawable Python base class:
- Created PyDrawable.h/cpp with base type (not yet inherited by UI types)
- Registered in module initialization

Closes #87 - Add visible property:
- Added bool visible=true to UIDrawable base class
- All render methods check visibility before drawing

Closes #88 - Add opacity property:
- Added float opacity=1.0 to UIDrawable base class
- UICaption and UISprite apply opacity to alpha channel

Closes #89 - Add get_bounds() method:
- Virtual method returns sf::FloatRect(x,y,w,h)
- Implemented in Frame, Caption, Sprite, Grid

Closes #98 - Add move() and resize() methods:
- move(dx,dy) for relative movement
- resize(w,h) for absolute sizing
- Caption resize is no-op (size controlled by font)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/UISprite.cpp

@@ -11,7 +11,13 @@ UIDrawable* UISprite::click_at(sf::Vector2f point)
     return NULL;
 }
 
-UISprite::UISprite() {}
+UISprite::UISprite() 
+: sprite_index(0), ptex(nullptr)
+{
+    // Initialize sprite to safe defaults
+    sprite.setPosition(0.0f, 0.0f);
+    sprite.setScale(1.0f, 1.0f);
+}
 
 UISprite::UISprite(std::shared_ptr<PyTexture> _ptex, int _sprite_index, sf::Vector2f _pos, float _scale)
 : ptex(_ptex), sprite_index(_sprite_index)
@@ -30,9 +36,21 @@ void UISprite::render(sf::Vector2f offset)
 
 void UISprite::render(sf::Vector2f offset, sf::RenderTarget& target)
 {
+    // Check visibility
+    if (!visible) return;
+    
+    // Apply opacity
+    auto color = sprite.getColor();
+    color.a = static_cast<sf::Uint8>(255 * opacity);
+    sprite.setColor(color);
+    
     sprite.move(offset);
     target.draw(sprite);
     sprite.move(-offset);
+    
+    // Restore original alpha
+    color.a = 255;
+    sprite.setColor(color);
 }
 
 void UISprite::setPosition(sf::Vector2f pos)
@@ -84,6 +102,28 @@ PyObjectsEnum UISprite::derived_type()
     return PyObjectsEnum::UISPRITE;
 }
 
+// Phase 1 implementations
+sf::FloatRect UISprite::get_bounds() const
+{
+    return sprite.getGlobalBounds();
+}
+
+void UISprite::move(float dx, float dy)
+{
+    sprite.move(dx, dy);
+}
+
+void UISprite::resize(float w, float h)
+{
+    // Calculate scale factors to achieve target size
+    auto bounds = sprite.getLocalBounds();
+    if (bounds.width > 0 && bounds.height > 0) {
+        float scaleX = w / bounds.width;
+        float scaleY = h / bounds.height;
+        sprite.setScale(scaleX, scaleY);
+    }
+}
+
 PyObject* UISprite::get_float_member(PyUISpriteObject* self, void* closure)
 {
     auto member_ptr = reinterpret_cast<long>(closure);