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>

TASKS.md

@@ -312,3 +312,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T26b | 1 | FAIL | a shaped crop's corner ignored the pointer's move up or down |
 | T26b | 2 | PASS | |
 | T26c | 1 | FAIL | the caption faded with the controls; Space on a focused bar button acted twice; Command letters fired the player and the strip; a long caption, cue words and the scrubber's tip ran past their boxes at 280px |
+| T26c | 2 | FAIL | the bar's fade restarted an animation id per rest and wake, which rebuilt the scrubber's drag, hover and focus under it |