Keep a button at its label's width in a stack and a form

<x-stack> stretches its children across by default and <x-form> is a
one-track grid, so a button written directly in either drew as a
full-width pill. M3 keeps a button's width "dynamic to fit label" and
says not to "stretch buttons into long flat shapes on large windows".
A button there now sits at the start edge at its own width; a stack
aligned start, centre or end is untouched, and an application's own
width still wins (a cap on the button itself was rejected for exactly
that: it would have limited an app's inline-size, which CascadeTest
guards). Chromium test in both directions, failing without the rules.
Decided with the user (plan step 46).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

UPGRADE.md

@@ -85,6 +85,8 @@ over a colour, use the role directly.
 - `<x-list dividers>` draws a 16px-inset rule; `<x-list selectable>` or `selection="single|multi"`
   makes it a `listbox` of `option`s with `aria-selected`.
 - `<x-card>` changes elevation on hover instead of its corner; `data-md-card` carries the variant.
+- A button directly in `<x-stack>` (stretched, the default) or `<x-form>` keeps its label's width at
+  the start edge instead of filling the width, as M3 asks; a full-width submit is your own CSS.
 - `<x-modal>` pins its header and actions and scrolls only the body; a `box-class` that set
   `overflow` no longer applies.
 - `<x-table>` rows are 52px and the automatic fine-pointer density is gone: pass `dense` for 36px