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>

tests/Feature/Components/StackTest.php

@@ -41,3 +41,10 @@ it('draws the stack from a stylesheet in the layout layer, imported by all.css',
         ->toContain(":where([data-md-stack]) {\n        --md-gap: 0px;")
         ->and((string) file_get_contents(__DIR__.'/../../../resources/css/all.css'))->toContain("@import './layout/stack.css';");
 });
+
+it('keeps a button at its label\'s width while the stack stretches the rest', function () {
+    // M3: keep a button's width "dynamic to fit label"; don't "stretch buttons into long flat shapes".
+    $css = (string) file_get_contents(__DIR__.'/../../../resources/css/layout/stack.css');
+
+    expect($css)->toContain("[data-md-stack]:is(:not([data-md-align]), [data-md-align='stretch']) > [data-md-button] {\n        align-self: start;\n    }");
+});