test(island): the pass's aim is read once the band stands at it

[ExtraToast]

Why

aimedAt reads the height a pass is aiming at off the band's inline style, and takes the first
non-null figure it sees. That figure is not always the one the pass is aiming at.

carry() in BandSwipe.vue pins the height it is leaving a tick before it can measure the one
it is arriving at, so for a moment the inline height is the departing stop's. A stop arriving
without a gesture is read in exactly that moment.

What this achieves

The helper reads the figure the pass aims at, rather than whichever figure happens to be pinned
when it looks. Measured on the board page: from=556 swiped=914 hit=914 stands=681 — that hit
should have been 681, and the assertion comparing them passed only because both were wrong the
same way.

How

The reader is told the height the band already stood at, and waits past it. Deterministic: no
timeout raised, no bare wait, and the assertions themselves are untouched.

Not in scope

The Playwright upgrade for #852, which is the pull request that follows this one and needs this
one merged first — without it, the upgraded Chromium reads the departing height and the case
fails with Received: 0.

Worth a reviewer's attention

This is a pre-existing race, not something a newer browser introduced. It reproduces on the
Playwright this repository pins today: the case it fixes was green in 5 of 5 full-file runs on
1.60.0, and the fix holds there too, so this stands on its own rather than as preparation.

Verification

yarn typecheck, yarn lint and yarn test:unit (1540 tests over 179 files) clean, all exit 0.

boards-swipe.motion.spec.ts in the motion project, five full-file runs on the current
Playwright: 9/9, 8/9, 9/9, 9/9, 9/9. The single failure was travels the line to a board a finger arrived at, which this change does not touch and which never calls aimedAt — it passed 6 of 6
when re-run alone, and reproduces on unmodified main. The known starvation flake, 8 workers on 4
vCPUs.

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Diff breakdown — █ added ░ removed, scaled to the largest row.

frontend                                           +12     -6    2
  e2e tests          █████████████████░░░░░░░░░    +12     -6    2

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production                                          +0     -0
tests                                              +12     -6
total (hand-written)                               +12     -6  2 files