Match only the scaffold's own main, and correct its header

Plan step 36, navigation review. `[data-md-scaffold] main` carried only
step 35's `--md-layout-margin`; the rewrite added the region's flex,
padding and overflow clip to it, which would also land on a <main> an
application nests in the page. It is `[data-md-scaffold-content] > main`
now. The header said panes read `--material-margin` (only the region
does), that the bar wrapper carries `--material-bottom-bar` (the root
does), and that the old utility and the hidden-bar override both sit in
material.components (the scaffold's is material.layout).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Feature/Components/ScaffoldTest.php

@@ -265,7 +265,9 @@ it('places the FAB at the bottom-end corner, clear of the bar and of a snackbar,
         ->toContain('+ var(--material-snackbar-height, 0px)')
         ->toContain('+ min(var(--material-snackbar-height, 0px), var(--md-sys-measurement-space200))')
         // The content region already has the window margin, so a pane inside draws none.
-        ->toContain("[data-md-scaffold] main {\n        --md-layout-margin: 0px;")
+        // Only the region's own <main>: one an application nests in the page takes none of this.
+        ->toContain("[data-md-scaffold-content] > main {\n        --md-layout-margin: 0px;")
+        ->not->toContain('[data-md-scaffold] main')
         ->and((string) file_get_contents(__DIR__.'/../../../resources/css/layout.css'))->toContain("@import './layout/scaffold.css';");
 });