feat(bankai): Phase 4 follow-ups — css convention, live titles, prerender, HTTP/2, size budget

- style.css convention: <entry dir>/style.css is imported by the virtual
  client entry (CSS is a client concern; server code never sees it — the
  v8 answer to sheetify), extracted and hashed by Vite, stylesheet-linked
  in the head and preloaded via Early Hints. Counter example styled.
- SSR <title> from state: the server reads the first stream chunk before
  writing the head, by which point stores and the first render slice have
  run — DOMTitleChange emits land in the document title. Counter emits.
- bankai build --prerender /,/about renders routes through the same
  toStream path to static <route>/index.html (precompressed, precached
  by the service worker, served with Early Hints).
- bankai serve --h2: HTTP/2 with a generated local cert (openssl,
  cached; allowHTTP1) — browsers only act on Early Hints over h2/h3.
  Tested with a real h2 client observing the 103 interim response.
- npm run size: the framework wire-size budget, enforced in CI. Whole
  framework (core + html engine + morph + hydrate) is 7.97 kB min+gzip /
  7.15 kB brotli; budget 8.5 kB with the v7 '4kb' context documented
  (that number excluded the html engine, which lived in the browserify
  transform).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014NgfSjHE11oFpoSnLVKLXd

packages/bankai/lib/plugin.js

@@ -9,7 +9,7 @@ const RESOLVED_CLIENT_ID = '\0' + CLIENT_ID
 // middleware ('\0' encodes as '__x00__')
 export const CLIENT_URL = '/@id/__x00__' + CLIENT_ID
 
-export default function bankaiPlugin (entry) {
+export default function bankaiPlugin (entry, { css } = {}) {
   return {
     name: 'bankai',
     resolveId (id) {
@@ -18,6 +18,10 @@ export default function bankaiPlugin (entry) {
     load (id) {
       if (id === RESOLVED_CLIENT_ID) {
         return [
+          // the style.css convention: CSS is a client concern, so the
+          // client glue imports it and server code never sees it — the
+          // v8 answer to what sheetify transforms did in v9
+          ...(css ? [`import ${JSON.stringify(css)}`] : []),
           `import create from ${JSON.stringify(entry)}`,
           'const app = typeof create === "function" && !create.mount ? create() : create',
           'app.mount(app.selector || "body")'
@@ -26,3 +30,16 @@ export default function bankaiPlugin (entry) {
     }
   }
 }
+
+// <entry dir>/style.css, when it exists
+export async function findCss (entry) {
+  const { access } = await import('node:fs/promises')
+  const { join, dirname } = await import('node:path')
+  const candidate = join(dirname(entry), 'style.css')
+  try {
+    await access(candidate)
+    return candidate
+  } catch (e) {
+    return null
+  }
+}