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/trigger_area.gd

@@ -0,0 +1,50 @@
+class_name TriggerArea
+extends Node2D
+## TriggerArea - Placeable rectangular sensor for the Sandbox Stage (Phase 4).
+##
+## A drawn Node2D (NOT a physics Area2D) that the stage's geometric event engine
+## polls for movable overlap. Purely visual plus a size query; no signals, no
+## physics. Draws a translucent green fill with a dashed green border so it reads
+## as a trigger zone in EDIT mode. World-space child, so no zoom division is
+## needed for the dashed border.
+
+## Half-extents of the rectangular sensor in local space.
+@export var size: Vector2 = Vector2(96.0, 96.0):
+	set(value):
+		size = value
+		queue_redraw()
+
+
+## Local-space rectangle (centered on the node origin) used for overlap tests.
+func get_area_rect() -> Rect2:
+	return Rect2(-size * 0.5, size)
+
+
+func _draw() -> void:
+	var rect := get_area_rect()
+	draw_rect(rect, Color(0.2, 0.8, 0.3, 0.12), true)
+	_draw_dashed_rect(rect, Color(0.2, 0.8, 0.3, 0.6), 2.0, 6.0, 4.0)
+
+
+## Dashed border along each edge of `rect` (top/right/bottom/left), drawn with a
+## small manual dash loop using draw_line.
+func _draw_dashed_rect(rect: Rect2, color: Color, width: float, dash: float, gap: float) -> void:
+	var tl := rect.position
+	var tr := rect.position + Vector2(rect.size.x, 0.0)
+	var br := rect.position + rect.size
+	var bl := rect.position + Vector2(0.0, rect.size.y)
+	_draw_dashed_edge(tl, tr, color, width, dash, gap)
+	_draw_dashed_edge(tr, br, color, width, dash, gap)
+	_draw_dashed_edge(br, bl, color, width, dash, gap)
+	_draw_dashed_edge(bl, tl, color, width, dash, gap)
+
+
+func _draw_dashed_edge(from: Vector2, to: Vector2, color: Color, width: float, dash: float, gap: float) -> void:
+	var dir := from.direction_to(to)
+	var total := from.distance_to(to)
+	var dist := 0.0
+	while dist < total:
+		var start := from + dir * dist
+		var len := minf(dash, total - dist)
+		draw_line(start, start + dir * len, color, width, true)
+		dist += dash + gap