test(queue): pin the panel's mouse model, and bound the highlight

Single click selects, ctrl and shift extend, double click plays from
that row — all four already worked, and nothing in either tier pinned
any of them, which is why the report could be made and could not be
settled. `queue-reorder.spec.ts` covers the keyboard and
`queue-overlay.spec.ts` the panel's mode; the pointer path had no
coverage at all, so "selection is broken here" and "selection is fine
here" were equally consistent with a green suite.

Measured with real mouse events rather than dispatched ones, because a
synthetic click aimed at the row bypasses the only thing that could be
swallowing it: click row 1 selects 1, ctrl+click 4 gives 1 and 4,
shift+click 7 extends to 1,4,5,6,7, a plain click collapses to one, and
a double click on row 3 leaves the backend playing row 3.

The three candidates the issue lists are all answered. The repaint was
already correct, and already correct on the day the issue was filed.
`resolveTrackIndexFromEvent` reads data-index, and DOM order matches
data order. A row control does swallow the click — `explore-link` stops
propagation on purpose, so a click on a name navigates and selects
nothing — but a hit-scan across a row makes the queue 12% link against
the track list's 21%, so the panel called broken is *less* covered by
links than the list called correct. That measurement killed the fix
this started out as.

Two traps are written into the spec because both faked a defect while
measuring. Fixture tracks are 2 seconds, so "double click row 3" read a
moment later reports whatever auto-advance moved on to — recorded twice
as an off-by-one that is not one, which is what `LONG_TRACK` exists
for. And the selection assertions are bounded at 500ms rather than
polled with the default 5s: `queue-panel` repaints two ways, the
explicit `requestUpdate()` and a per-render `keyFunction` arrow, and
with *both* removed the highlight still arrives — at 134ms, 3.9s and
5.8s against 5-17ms healthy. Four seconds is indistinguishable from
broken to a user and invisible to a generous poll.

Mutation-tested rather than trusted: `playAtIndex(index + 1)` fails both
double-click tests, treating every click as ctrl+click fails both
selection tests, and removing both repaint mechanisms fails all three
selection tests — the last only because of the bound.

Closes #43

frontend/src/components/queue-panel/queue-panel.ts

@@ -277,6 +277,26 @@ export class QueuePanel
         return this.queue.tracks.length;
     }
 
+    /**
+     * Repaint the rows when the selection changes.
+     *
+     * `<lit-virtualizer>` renders through the `virtualize` directive,
+     * which reacts to its *own* properties and not to the host having
+     * re-rendered, so host state like a selection reaches the rows only
+     * if it is pushed. `track-list` has always done this and both
+     * playlist views had to be taught it.
+     *
+     * **There is a second, accidental mechanism here and it must not be
+     * mistaken for this one**: `.keyFunction` below is a per-render
+     * arrow, so it is a changed property on every host update and
+     * repaints the rows by itself. Removing *either* alone changes
+     * nothing observable, which is why #43 could not be settled by
+     * reading the code. With both gone the highlight still arrives —
+     * on whatever unrelated render happens next, measured at 134ms,
+     * 3,866ms and 5,816ms against 5–17ms healthy, which a user cannot
+     * tell from broken. `queue-selection.spec.ts` asserts the
+     * *promptness* rather than the eventual state for that reason.
+     */
     onSelectionChanged(): void {
         this.virtualizer?.requestUpdate();
     }