Inset rings packed into the shadow tokens are erased by any opaque child (BottomSheet hit this; the pattern is systemic)

Every bundled theme adds a hairline inner ring to the elevation shadow tokens, dark mode only. Neutral's:

```
'--shadow-high':
'0 4px 6px light-dark(oklch(0 0 0 / 10%), oklch(0 0 0 / 50%)), ' +
'0 12px 24px light-dark(oklch(0 0 0 / 15%), oklch(0 0 0 / 70%)), ' +
'inset 0 0 0 1px light-dark(transparent, oklch(1 0 0 / 15%))'
```

The intent is good — in dark mode a drop shadow is black on near-black and gives an elevated surface no edge, so the ring supplies one. But the mechanism is fragile in a way that isn't visible from the theme file.

## The problem

**An inset shadow paints above the element's own background but below its children.** So any opaque child laid flush to the inner edge erases the ring for exactly as far as it extends. `Section` — the standard content wrapper, opaque `--color-background-surface` in its default variant — does precisely that.

The failure mode is worse than "no ring": it's a *partial* ring. Content covers the top of the surface, empty space below it does not, so the ring switches on partway down and reads to a viewer as the border changing width. Measured on the docs site, BottomSheet's left edge in dark mode (neutral theme):

| | outside | edge | just inside |
|---|---|---|---|
| above the content end | `2,2,2` | `60,60,60` | `38,38,38` |
| below the content end | `2,2,2` | `60,60,60` | **`71,71,71`** |

This is what a user reported as "the bottom sheet's border width is not consistent."

## Why a component can't fix it properly

The ring is inside a shadow token, so from a component there is no way to read it separately, lift it above children, or opt a child out. The only local moves are: hide it evenly, leave it half-covered, or make content wrappers transparent (which breaks Section's contract). PR #5305 takes the first — the BottomSheet's scrolling body paints the surface across the whole inner box, hiding the ring, and the sheet draws its own `--color-border` hairline instead. That fixes the symptom for one component and leaves the sheet's edge treatment diverging from the theme's other elevated surfaces.

## Scope

Any elevated surface whose direct content can be opaque and flush to its edge — `Dialog` (same `--shadow-high`, and its inner wrapper sets no background), `Popover`, `HoverCard`, `DropdownMenu`, `Card` with a full-bleed opaque child. Confirmed on BottomSheet; I did not measure the others (the Dialog docs example is fullscreen, and a top-layer dialog didn't pick up the themed shadow in my local harness, so that one needs checking in a real app).

`Card` is the interesting counter-example: it composes `--_card-elevation` and `--_card-ring` into its box-shadow list *and* draws a real `borderWidth`/`borderColor`. A component that owns a border is immune, because a border paints outside the padding box where no child can reach.

## Options worth discussing

1. **A dedicated `--surface-ring` token** that components apply as a border (or outline), not a shadow. Immune to children by construction; themes keep control of colour and width. Costs a token and a pass over the elevated surfaces.
2. **Themes stop putting rings in shadows**, and dark-mode edge definition comes from `--color-border` on each surface. Simplest model; means the elevated surfaces need a border they mostly don't have today.
3. **Status quo plus a documented rule**: "an elevated surface must not let an opaque child reach its inner edge," enforced by each surface painting its own background across its inner box (what #5305 does). Cheapest, but it's a rule nobody will remember, and it silently defeats the theme's ring.

I lean 1 — it keeps the theme's intent, works with an arbitrary child, and matches what Card already does by hand.

Related: #5305 (the BottomSheet fix that motivated this), #5014 (overriding one side of a colour pair voids the contrast guarantee), #2150 (dark mode research/guidelines).

Start by inspecting the bundled theme shadow tokens and the elevated surfaces named here: BottomSheet, Dialog, Popover, HoverCard, DropdownMenu, and Card. Reproduce the dark-mode BottomSheet edge behavior, then compare how Card composes its ring and border. Done requires choosing and applying a consistent edge-treatment approach across affected surfaces, with the behavior verified for opaque children.

Written by the indexing model from the issue text.