Apply new waveform colors without restarting SE (#13897)

[niksedk]

Fixes #13897 — changing the waveform color or the selected waveform color only took effect after restarting SE, while every other waveform color changed instantly.

Root cause

The fancy draw style (the default) batches waveform columns by a quantized amplitude bucket and caches a pen per bucket. The bucket key carries no color at all, so:

• _fancyWaveformPenCache / _fancyWaveformGradientCache / _fancyWaveformGlowPenCache hold pens painted in whatever color was current when that bucket was first drawn, and
• _fancyBatches — the pooled batch dictionary, which keeps a Pen of its own — does too.

ResetCache() cleared the three pen caches but not _fancyBatches. So Settings -> OK did everything right (pushed the new colors, reset the caches, and the geometry cache key does include the colors), the geometry was rebuilt — and then stroked with the stale batch pens. Only a brand-new control, i.e. a restart, dropped them.

That is exactly the split reported: waveform color, selected color (and fancy high color, which the reporter probably did not try) are the only three that go through the fancy batch/pen cache. Background, cursor, shot change, paragraph left/right and text build a fresh brush in their setter and are drawn live, so they always updated at once. The classic draw style was never affected either — it uses _paintWaveform / _paintPenSelected, rebuilt in the setters.

Fix

• New ResetFancyColorCaches() clears all four color-bearing caches together, and invalidates the visual (the color properties are not AffectsRender, so nothing else asked for the repaint).
• Called from ResetCache() and from the WaveformColor / WaveformSelectedColor / WaveformFancyHighColor setters, so no caller has to remember to reset anything. WaveformFancyHighColor was a plain auto-property with no invalidation at all.

Tests

tests/UI/Controls/AudioVisualizerColorTests.cs renders a fancy waveform, changes a color, re-renders, and asserts the cached draw pens no longer carry the old color — for the waveform color (with and without an explicit ResetCache()), the selected color, and the fancy high color.

All 4 fail on main and pass with the fix; the full UI suite (3246 tests) passes.