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`.

AGENTS.md

@@ -143,7 +143,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - gpui styles a drag-over only on an element with its own hitbox. A veil that shows while files hover takes the drop itself (`chat::DragDropOverlay`).
 - gpui 0.2.2 never clears `debug_bounds`: a test sees an element appear, not leave.
 - The system time zone comes from `typography::format::system_zone`, which fails loud; jiff's `TimeZone::system()` falls back to UTC in silence.
-- A list opens with its cursor on the current choice: Select on the chosen row, Combobox on its value, MultiSelect on the first one ticked.
+- A list opens with its cursor on the current choice: Select on the chosen row, Combobox on its value, MultiSelect on the first one ticked, SelectableList on its first selected row. A selection the owner makes, not the echo of the list's own pick, moves SelectableList's cursor there.
 - A search step, a tool call or an agent's step says where it stands with `chat::StepState` and draws it with `chat::step_mark`.
 - A flex line whose free space goes to an auto margin loses its gaps in taffy 0.9. A row that parts left and right groups its sides and uses `justify_between`; `ml_auto` is safe only beside a `flex_1` item that leaves no free space.
 - A header that holds a name beside counts and actions wraps: `flex_wrap`, the name `flex_1` with `min_w(label_width)`, the rest one `flex_none` group that drops below.