Media: the player's bar fades by elapsed time, so its scrubber and buttons keep their state

The fade restarted an animation id at each rest and wake, and gpui hands
that id to every descendant: the scrubber's drag, hover and focus handle
and the buttons' focus were rebuilt under it. The bar's opacity now reads
the time since the last change through motion::since_change. The stage
tracks focus even without key handlers, so a focused bar button keeps
the bar up. The player's tests move to their own file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -39,7 +39,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - One component, one home. Duplicates point to it.
 - Fail fast. No silent fallbacks. Log state changes with `log`.
 - Files stay at or under 500 lines. Comments are one short line, and rare.
-- A repeating animation (spinner, breath, sweep) holds still under reduced motion instead of shortening, or it would spin at 1 ms a turn. An effect set off by a change (flash, shake) reads elapsed time rather than restarting an animation id, so what it holds keeps its state.
+- A repeating animation (spinner, breath, sweep) holds still under reduced motion instead of shortening, or it would spin at 1 ms a turn. An effect set off by a change (flash, shake, a bar that fades at rest) reads elapsed time through `motion::since_change` rather than restarting an animation id, so what it holds keeps its state: an animation's id passes to every descendant, and a new one rebuilds their focus, drags and hover.
 - `motion::duration()` honors reduced motion. Springs overshoot: use them inside animators, never as gpui easings, because gpui asserts eased values stay in `0..=1`.
 - `svg()` paints only with its own `text_color`. `Icon` always sets one.
 - Buttons stay out of focus on click (`prevent_default` on mouse down). Tab still reaches them.