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

CLAUDE.md

@@ -1402,7 +1402,7 @@ descendants. On the reference device the main panel spans 0-318 of a
 items cut off, with no way to reach them. `showModal()` is Chrome 37
 and uses the real top layer, so a dialog is immune by construction.
 
-Six things about it are load-bearing.
+Seven things about it are load-bearing.
 
 **"Dialogs are fine" needed checking, because every other dialog in
 this app is mounted in `index.html`** — outside `.main-panel` — so it
@@ -1431,6 +1431,25 @@ doing nothing, which reads as the gesture breaking. `menu-dismiss` is
 that signal; the three surfaces that do not use `ContextMenuController`
 bind it themselves.
 
+**A sheet that scrolls says so, and `background-attachment` is what
+asks whether it does** (#207). The sheet is capped at 85vh — a surface
+covering the whole screen is a page, not a sheet — so a long menu's
+body scrolls, and for three phases it scrolled *silently*: measured at
+424x439, eight items ended at y=470 with the fold at 439, and where the
+cut lands on a row boundary the sheet ends in a clean edge that reads
+as the end of the list. The fade is two background layers on
+`wa-dialog::part(body)` — 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 it 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. **The curve is steep because the rows under it stay
+live**: a scrim over a menu item is that item's text surface, and the
+4.5:1 rule applies to it — 32px already down to a quarter strength at
+14px spends its weight below the last legible label, measured at 9.9:1
+on the light ramp, whose `bgElevated` is `#e9ecef`.
+
 **The playlist submenu is a sheet too, and it had to be.** It is a
 `placement="right-start"` flyout, and making the menu full-width moved
 its anchor — measured at x −182 to 0, entirely off-screen, so "Add to