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/stickman_rig.gd

@@ -272,7 +272,7 @@ signal state_changed(new_state: int)
 # Director signals (Phase 3a)
 # ---------------------------------------------------------------------------
 
-signal arrived                                  # walk_to reached its destination
+signal arrived(target: Vector2)                 # walk_to reached its destination
 signal action_started(action: Dictionary, index: int)
 signal action_finished(action: Dictionary, index: int)
 signal queue_finished                            # queue ran to completion (not on stop)
@@ -1134,7 +1134,7 @@ func _finish_walk(reason: String = "") -> void:
 	_restore_standing_markers()
 	_walk_done = true
 	_walking = false
-	arrived.emit()
+	arrived.emit(_walk_target_feet)
 
 
 func _cancel_walking() -> void:
@@ -1215,6 +1215,24 @@ func queue_size() -> int:
 	return action_queue.size()
 
 
+## Append `actions` to the queue and, when the runner is idle, resume execution
+## at the first newly-appended action WITHOUT replaying the existing queue.
+## Used by the Phase 4 event engine to inject reactive actions onto a stickman.
+func enqueue_reactive(actions: Array[Dictionary]) -> void:
+	if actions.is_empty():
+		return
+	var start := action_queue.size()
+	action_queue.append_array(actions)
+	queue_changed.emit()
+	if _runner_state == RunnerState.IDLE:
+		# Resume at the first appended action (not at index 0), so any queued
+		# sequential actions are skipped over rather than replayed.
+		_current_index = start - 1
+		_runner_state = RunnerState.EXECUTING
+		_action_phase = ActionPhase.NONE
+		_stop_requested = false
+
+
 # ---------------------------------------------------------------------------
 # Runner state machine (Phase 3a)
 # ---------------------------------------------------------------------------