feat: Implement Phase 4 Trigger Events System

- Added a new event-driven system for reactive storytelling, allowing rules like "When X happens, do Y."
- Introduced TriggerArea class for placeable sensors in the stage.
- Enhanced StickmanRig to emit signals for actions and arrivals.
- Updated StageDirectorVisuals to render rules visually with labels and badges.
- Modified StageSpawner to support spawning TriggerAreas.
- Improved text baseline calculations in speech bubbles and rule labels.
- Added tests for text baseline fixes to ensure proper rendering.
- Documented the implementation plan for Phase 4 in PHASE_4_TRIGGER_EVENTS.md.
- Created a polish plan for Phase 4 in PHASE_4b_POLISH.md.

scripts/prop_block.gd

@@ -88,6 +88,14 @@ var _collision_polygon: CollisionPolygon2D
 var _collision_shape: CollisionShape2D
 var _circle_shape: CircleShape2D
 
+# ---------------------------------------------------------------------------
+# Signals
+# ---------------------------------------------------------------------------
+
+## Emitted when this prop's body makes contact with another physics body.
+## `other` is the body that entered contact (Phase 4 trigger source).
+signal collided(other: Node)
+
 # ---------------------------------------------------------------------------
 # Lifecycle
 # ---------------------------------------------------------------------------
@@ -96,6 +104,15 @@ func _ready() -> void:
 	_ensure_children()
 	_apply_shape()
 	_apply_style()
+	# Enable contact reporting so the body_entered signal fires (Phase 4).
+	contact_monitor = true
+	max_contacts_reported = 8
+	if not body_entered.is_connected(_on_body_entered):
+		body_entered.connect(_on_body_entered)
+
+
+func _on_body_entered(body: Node) -> void:
+	collided.emit(body)
 
 # ---------------------------------------------------------------------------
 # Internal build / apply