fix(ui): guard empty text in show_monologue — no ghost slots or queue entries (#122)

show_monologue() now returns early when text.is_empty(), preventing:
- ghost visible slots with blank labels
- stagger timer advancing on empty calls
- empty strings queuing when slots are full

Tests: add test_show_monologue_with_empty_text_does_not_set_displaying and
two companion cases (stagger timer unchanged, no enqueue when full) anticipating
Hoshe's QA additions to the test suite.

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

client/tests/test_monologue_display.gd

@@ -46,6 +46,36 @@ func test_stagger_timer_zero_on_init() -> void:
 	d.queue_free()
 
 
+func test_show_monologue_with_empty_text_does_not_set_displaying() -> void:
+	# Empty text must be ignored — no visible slot created, no queue entry.
+	# Prevents ghost display nodes and stagger timer contamination.
+	var d = _make_display()
+	if d == null: return
+	d.show_monologue("", 5.0)
+	assert_int(d._visible.size()).is_equal(0)
+	assert_int(d._queue.size()).is_equal(0)
+	d.queue_free()
+
+
+func test_empty_text_does_not_advance_stagger_timer() -> void:
+	var d = _make_display()
+	if d == null: return
+	d.show_monologue("", 5.0)
+	assert_float(d._next_fade_in_msec).is_equal(0.0)
+	d.queue_free()
+
+
+func test_empty_text_when_slots_full_does_not_enqueue() -> void:
+	var d = _make_display()
+	if d == null: return
+	d._next_fade_in_msec = 0.0; d.show_monologue("A", 10.0)
+	d._next_fade_in_msec = 0.0; d.show_monologue("B", 10.0)
+	d._next_fade_in_msec = 0.0; d.show_monologue("C", 10.0)
+	d.show_monologue("", 5.0)
+	assert_int(d._queue.size()).is_equal(0)
+	d.queue_free()
+
+
 # ---------------------------------------------------------------------------
 # Single-line display
 # ---------------------------------------------------------------------------