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/build.js

@@ -4,28 +4,33 @@
 //   dist/assets/*            hashed client chunks + css
 //   dist/.vite/manifest.json Vite's build manifest
 //   dist/bankai.json         bankai's route/asset metadata for serving
+//   dist/**/index.html       prerendered routes (--prerender)
 //   dist/sw.js               service worker (if <entry dir>/sw.js exists),
 //                            with the precached asset list defined in
 //   *.br / *.gz              precompressed siblings for every text asset
 
-import { readFile, writeFile, readdir, stat, access } from 'node:fs/promises'
+import { readFile, writeFile, readdir, stat, access, mkdir } from 'node:fs/promises'
 import { join, dirname, resolve } from 'node:path'
+import { pathToFileURL } from 'node:url'
 import { brotliCompress, gzip, constants } from 'node:zlib'
 import { promisify } from 'node:util'
 
-import bankaiPlugin, { CLIENT_ID } from './plugin.js'
+import bankaiPlugin, { findCss, CLIENT_ID } from './plugin.js'
+import { resolveApp } from './app.js'
+import { documentHead, documentTail } from './document.js'
 
 const brotli = promisify(brotliCompress)
 const gzipP = promisify(gzip)
 
-export default async function buildApp ({ entry, outDir = 'dist', title = 'choo' }) {
+export default async function buildApp ({ entry, outDir = 'dist', title = 'choo', prerender = [] }) {
   const { build } = await import('vite')
   entry = resolve(entry)
   outDir = resolve(outDir)
+  const css = await findCss(entry)
 
   await build({
     appType: 'custom',
-    plugins: [bankaiPlugin(entry)],
+    plugins: [bankaiPlugin(entry, { css })],
     logLevel: 'warn',
     build: {
       outDir,
@@ -48,11 +53,36 @@ export default async function buildApp ({ entry, outDir = 'dist', title = 'choo'
   const meta = { title, entry, assets, builtAt: new Date().toISOString() }
   await writeFile(join(outDir, 'bankai.json'), JSON.stringify(meta, null, 2))
 
+  // static prerender: routes rendered through the same toStream path the
+  // server uses, written as <route>/index.html
+  const prerendered = []
+  if (prerender.length) {
+    const mod = await import(pathToFileURL(entry))
+    for (const route of prerender) {
+      const app = resolveApp(mod, entry)
+      const state = {}
+      let body = ''
+      const decoder = new TextDecoder()
+      for await (const chunk of app.toStream(route, state)) {
+        body += decoder.decode(chunk, { stream: true })
+      }
+      const page = documentHead({ title: state.title || title, ...assets }) + body + documentTail(state)
+      const rel = route === '/' ? 'index.html' : join(route.replace(/^\//, '').replace(/\/$/, ''), 'index.html')
+      const target = join(outDir, rel)
+      await mkdir(dirname(target), { recursive: true })
+      await writeFile(target, page)
+      prerendered.push(route)
+    }
+  }
+
   // service worker: <entry dir>/sw.js, built standalone with the asset
   // list injected — the choo-service-worker convention, manifest edition
   const swSource = join(dirname(entry), 'sw.js')
   if (await exists(swSource)) {
-    const precache = ['/', ...assets.scripts, ...assets.modulepreload, ...assets.css]
+    const precache = [
+      ...(prerendered.length ? prerendered : ['/']),
+      ...assets.scripts, ...assets.modulepreload, ...assets.css
+    ]
     await build({
       appType: 'custom',
       logLevel: 'warn',
@@ -71,7 +101,7 @@ export default async function buildApp ({ entry, outDir = 'dist', title = 'choo'
   // precompress text assets so the server never compresses at request time
   const compressed = await precompress(outDir)
 
-  return { outDir, meta, compressed }
+  return { outDir, meta, compressed, prerendered }
 }
 
 function collectCss (manifest, entryChunk) {