P10.4: snapshot-test cascade-cue depth->index mapping (sx)

Add tests/cascade_cue.sx, a headless logic test that reuses audio.sx's
pure cascade_cue_index / cascade_cue_name (the exact functions
play_cascade calls) to lock the cascade-depth -> combo-cue clamp:
depth <= 1 -> combo1, depth >= COMBO_CLIPS -> combo5, monotonic between.
Covers the escalation logic the audio playback path can't gate-cover,
with no audio. Committed stdout/exit snapshot; suite is now 19 tests.

tests/expected/cascade_cue.exit

@@ -0,0 +1 @@
+0