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/ui/monologue_display.gd

@@ -66,7 +66,10 @@ func _process(delta: float) -> void:
 # Display a monologue line.
 # priority: higher number = more important (default 2; urgent beats normal).
 # is_urgent: visual flag — full opacity + elevated colour. Bloom deferred.
+# Empty text is silently ignored — no slot created, no queue entry.
 func show_monologue(text: String, duration: float, priority: int = 2, is_urgent: bool = false) -> void:
+	if text.is_empty():
+		return
 	var now := float(Time.get_ticks_msec())
 	if _visible.size() < MAX_VISIBLE and now >= _next_fade_in_msec:
 		_show_line(text, duration, priority, is_urgent)