fix: exit number rain loop when scene completes

[patrickschaper]

Problem

The stepNumberRain() function had an infinite loop that froze all screens during the number-to-rain scene transition in multi-display engine mode.

Root cause: When numberSceneDoneRequested was set to true by blackoutColumns(), the while condition activeScene == .numberIntro remained true forever because:

• activeScene never changed (stays .numberIntro in engine mode; only changes on next frame)
• blackoutColumns() returned immediately on subsequent iterations due to guard check
• The loop spun infinitely within a single advance() call, freezing the entire thread

Solution

Added !numberSceneDoneRequested check to both:

1. The while loop condition in stepNumberRain()
2. The guard statement in stepNumberRain()

This ensures the loop exits immediately when the scene signals completion, allowing the phase-advance barrier to resolve and transition to the rain scene.

Testing

• All 31 existing tests pass
• Manual testing on multi-display setup needed to verify sync behavior

Changes

• NativeMatrixRenderer.swift: Added done-flag checks to stepNumberRain()