Fix mania hold notes dimming unexpectedly

[smoogipoo]

Fixes #29223

This fixes several issues around hold note dimming.

Notice in the following video:

• The tail piece of the first note not getting dimmed.
• The body of the second note not responding to dimming at all.

2026-03-17.19-42-26.mp4

Then, notice in the following video:

• The body piece of the second note is dimmed from the very beginning.

2026-03-17.19-42-53.mp4

This requires a specific setup whereby the hold note and its components must be reused from the pool. In particular:

1. The hold note must be long. So long that by the time the tail becomes on screen, the body will already have dimmed.
2. The hold note must be re-used from the pool. We can induce this by setting the pool sizes to 1 in Column.cs.
3. The second hold note should be placed far enough in the timeline that the first hold note dies by the time it becomes visible.
4. Scroll speed should be adjusted to fit the above constraints.

I haven't done a full deep dive into exactly why this is happening, so the fix here is hand-wavy. That said, just by looking at the old code in LegacyBodyPiece you'll get a feeling that something's bound to go wrong;

• It never resets the missingFadeTime state.
• It never resets the colours back to Color4.White.
• It applies transforms onto external components.
• It jumps through hoops to figure out how to set missingFadeTime.

My hope is that these changes first bring some sanity in the process, and if it breaks again I'll consider doing a more proper root cause (I've had this issue in the back of my mind for about 1 year).

With this PR, they now behave as expected:

2026-03-17.19-43-59.mp4

2026-03-17.19-44-15.mp4