Lists: a selectable list's cursor starts on the owner's pick; the waveform's fit is tested

SelectableList keeps the owner's last selection and the one it last
sent. A selection the owner makes, on first paint or later, moves the
cursor to its first row; the echo of the list's own pick leaves the
cursor where the pick put it. The playlist's keys start from the song
under way. The waveform's bar count and inset move into `fit`, tested
beside `resample`.

TASKS.md

@@ -314,3 +314,4 @@ Per-component lines live in `tasks/`. Tags there:
 | 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 |
 | T26c | 3 | PASS | |
+| T26d | 1 | FAIL | the playlist's keys started from the first row, not the song under way; the waveform's bar fit sat untested in paint |