feat: add Vercel serverless function entry point

Exports a Node.js handler that delegates to the shared
request handler for SSR + API routes on Vercel.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rsbh

packages/chronicle/src/server/entry-vercel.ts

@@ -0,0 +1,26 @@
+// Vercel serverless function entry — built by Vite, deployed as catch-all function
+import type { IncomingMessage, ServerResponse } from 'http'
+import { readFileSync } from 'fs'
+import path from 'path'
+import { handleRequest } from './request-handler'
+
+const templatePath = path.resolve(__dirname, 'index.html')
+const template = readFileSync(templatePath, 'utf-8')
+
+export default async function handler(req: IncomingMessage, res: ServerResponse) {
+  const url = req.url || '/'
+  const baseUrl = `https://${req.headers.host || 'localhost'}`
+
+  try {
+    const response = await handleRequest(url, { template, baseUrl })
+
+    res.statusCode = response.status
+    response.headers.forEach((value: string, key: string) => res.setHeader(key, value))
+    const body = await response.text()
+    res.end(body)
+  } catch (e) {
+    console.error(e)
+    res.statusCode = 500
+    res.end((e as Error).message)
+  }
+}