fix(poc): resolve evaluator and review findings

- Add RectangleShape2D to Character and InteractiveObject collision areas
- Fix HUD button signal connections in _ready()
- Fix character_placed signal emitting global_position
- Extract DEFAULT_DRAG_RADIUS constant in DragDropComponent
- Type Variant on JSON parsed variable in SaveManager
- Extract music symbol constants in HUD
- Refactor duplicated drag input code in InputManager

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

scripts/systems/hud.gd

@@ -1,13 +1,12 @@
 ## HUD — heads-up display with back button and music toggle.
 extends CanvasLayer
 
+const MUSIC_ON_SYMBOL: String = "♪"
+const MUSIC_OFF_SYMBOL: String = "✕"
+
 var _music_enabled: bool = true
 
 
-func _ready() -> void:
-	pass
-
-
 func _on_back_button_pressed() -> void:
 	get_tree().quit()
 
@@ -18,4 +17,4 @@ func _on_music_toggle_pressed() -> void:
 	AudioManager.set_music_volume(volume)
 	var btn: Button = get_node_or_null("MusicToggle") as Button
 	if btn != null:
-		btn.text = "♪" if _music_enabled else "✕"
+		btn.text = MUSIC_ON_SYMBOL if _music_enabled else MUSIC_OFF_SYMBOL