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>