fix(ui): the phone's context sheet says when it scrolls

The bottom sheet's body has scrolled since #60 and said nothing about
it. Measured at 424x439, the track list's menu ended at y=470 with the
fold at 439 — reachable, since the body is `overflow-y: auto`, but with
no affordance saying so, and worst where the cut lands on a row
boundary and the sheet ends in a clean edge that reads as the end of
the list.

The cap stays: `menu-surface`'s own comment says a surface covering the
whole screen is a page, not a sheet. What changes is that the body
draws a fade, from two background layers whose *attachments* are the
feature — a shadow pinned to the box (`scroll`) under a cover of the
sheet's own colour painted at the end of the content (`local`), which
scrolls up over the shadow exactly when there is nothing more to see.
So the fade is absent on a menu that fits, present the moment one does
not, and gone again at the end of the list, with no scroll listener and
nothing reaching into `wa-dialog`'s shadow root for the scroller.
`background-attachment` is Chrome 4; the reference device is Chrome 113.

The other two options in the report — a shortened last row, or a max
height that makes the cut obvious — both need `height mod 48`, which
CSS cannot express, and the observed case is exactly the one where the
cut already lands on a row boundary.

The curve is steep rather than linear because the rows under it stay
live: a scrim over a menu item is that item's text surface, so the
4.5:1 rule reaches it, and the light ramp is what makes that real.
A 48px linear scrim at 0.8 greyed the last label to 5.0:1; 32px already
down to a quarter strength at 14px measures 9.9:1 there and spends its
weight on the strip below it.

The test asserts the pair of attachments rather than the pixels, on
this file's existing grounds that no tier here renders like the device
— it fails on the pre-fix stylesheet with `expected 'scroll' to be
'local, scroll'`. The rendered result was measured in the harness and
is recorded in `.planning/NOTES.md`.

Closes #207

.planning/NOTES.md

@@ -4912,3 +4912,49 @@ bridge leaves the wizard up with its "Get Started" button correctly
 disabled — it gates on a directory chosen *in the wizard*, and the
 existing-library check runs once, on mount. A reload clears it. Nothing
 is broken; it cost twenty minutes of believing a tap had been swallowed.
+
+## The sheet's scroll fade, and where a scrim may not go (measured 2026-08-23, headless)
+
+#207's answer. The affordance is two background layers on
+`wa-dialog::part(body)` and the conditionality is
+`background-attachment`, not a scroll listener: a cover of the sheet's
+own colour painted at the end of the *content* (`local`) over a shadow
+pinned to the box (`scroll`), so the cover scrolls up and hides the
+shadow exactly when there is nothing more to see.
+
+Measured at 424x360 (which is where a menu overflows on `main`, since
+`main` does not yet carry #67's eighth item — at 424x439 the track
+list's seven items are `scrollHeight` 364 against `clientHeight` 364,
+fitting exactly). Pixel at x=300, dark ramp, `bgElevated` `#343a40`:
+
+| y | before | more below | at the end of the list |
+|---|---|---|---|
+| 330 | 52,58,64 | 50,56,62 | 52,58,64 |
+| 340 | 52,58,64 | 43,48,53 | 52,58,64 |
+| 350 | 52,58,64 | 33,37,40 | 52,58,64 |
+| 359 | 52,58,64 | 22,24,27 | 52,58,64 |
+
+Three things worth keeping.
+
+**A menu that fits draws nothing**, which is the same measurement: at
+424x439 the sheet is flat 52,58,64 to its bottom edge, because with no
+overflow the `local` layer's positioning area *is* the padding box and
+the cover lands on top of the shadow.
+
+**A scrim over a menu row is that row's text surface**, so the 4.5:1
+rule reaches it and this is why the curve is steep rather than linear.
+A row is 48px with its label centred; 32px of scrim already down to a
+quarter strength at 14px puts about 0.06 at the label. Checked on the
+light ramp (`bgElevated` `#e9ecef`, text `#212529`) by overriding the
+two custom properties on `:root`: background at the label 205,207,210,
+which is **9.9:1**. The first draft — a linear 48px at 0.8 — put ~0.375
+on that label, 5.0:1, passing but visibly greyed. The bottom few pixels
+go to ~2.4:1 in either draft and are deliberately below where any
+label of a *fully visible* row sits; a label that lands there belongs
+to the half-cut row, which is the thing being signalled.
+
+**A dark scrim on a dark surface reads far worse in a shrunk screenshot
+than on screen.** The first two probes (24px/0.45, then 32px/0.75) were
+measurably present — 52,58,64 down to 30,33,37 — and invisible in the
+inline preview. Crop the bottom 70px and scale it up before judging;
+the pixel values are the honest answer either way.