Add Stylesheets::bundle(), a build-free CSS import resolver

Plan step 37: the showcase and the error page's fallback serve CSS
without the application's Vite build, so nothing deduplicates their
@imports for them the way Vite's bundled postcss-import does — a
browser's native @import fetches every occurrence, it does not skip a
file it already loaded. Stylesheets::bundle(array $files, ?string
$base = null): string does in PHP what that build step does: it
inlines every @import depth-first, each file once, first occurrence
kept; leaves a bare specifier or an absolute URL untouched; rewrites a
relative url() against $base (or, without one, against the directory
of $files[0]); breaks a cycle instead of looping; throws naming the
importer when an import is missing; and caches per resolved file list
and mtime, with resetCache() for tests.

tests/Feature/StylesheetsBundleTest.php covers bundle()'s own
behaviour (dedup, url() rewriting, cycles, the missing-import
exception, the cache) and, at the end, pins the Vite deduplication
bundle() is modelled on against a real build — of all.css alone
(tests/Fixtures/dedup.vite.config.mjs), whose docblock explains why:
the Workbench's own entry still shares one file with @tailwindcss/vite
until plan step 39 removes it, and that plugin bundles its whole
reachable module graph itself, without the same dedup guarantee.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Fixtures/dedup.vite.config.mjs

@@ -0,0 +1,26 @@
+import { defineConfig } from 'vite';
+
+// Builds resources/css/all.css alone, with no other plugin in the graph: it isolates Vite's own
+// bundled postcss-import, whose `skipDuplicates` option is the mechanism
+// docs/plans/material-3-alignment.md's "Tailwind's footprint" paragraph measured and
+// StylesheetsBundleTest.php pins against the real bundler — a stylesheet several files import
+// lands once, at its first position.
+//
+// The Workbench's own entry (vite.config.js) still shares one file with `@tailwindcss/vite` until
+// plan step 39 removes it, and that plugin bundles its whole reachable module graph itself,
+// independently of Vite's CSS pipeline; it does not carry the same `skipDuplicates` option, so a
+// shared component stylesheet currently lands once per import site there, not once overall. That
+// is a pre-existing limitation of mixing the two in one entry, not something plan step 37
+// introduces or can fix without moving Tailwind out of the Workbench's entry (step 39's job) — this
+// fixture is what actually exercises the claim being pinned, decoupled from it.
+export default defineConfig({
+    build: {
+        outDir: process.env.DEDUP_OUT_DIR,
+        emptyOutDir: true,
+        manifest: true,
+        rollupOptions: {
+            input: 'resources/css/all.css',
+        },
+    },
+    logLevel: 'silent',
+});