Add SSR bridge: Django template backend + Bun subprocess renderer

Mizan's SSR is a Django template backend. Configure in TEMPLATES:

    TEMPLATES = [{
        'BACKEND': 'mizan.ssr.MizanTemplates',
        'OPTIONS': {'worker_path': 'frontend/ssr-worker.tsx'},
    }]

Then render(request, 'ProfilePage', {'user_id': 5}) renders the React
component via a persistent Bun subprocess. The component name is the
template name. The context dict becomes props.

Architecture:
- Bun worker: stdin/stdout JSON-RPC, renderToString, component registry
- Django bridge: subprocess lifecycle, crash recovery, concurrent renders
- Template backend: implements Django's BaseEngine interface

This is the AFI's SSR boundary:
- Backend adapter implements mizan.ssr() (data gathering)
- Frontend adapter implements renderToHTML() (component rendering)
- Bun subprocess is the runtime hosting the frontend adapter

11 tests: ping, render, error handling, crash recovery, concurrent
renders (5 threads), template backend integration. All require Bun.

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

packages/mizan-ssr/src/index.ts

@@ -0,0 +1 @@
+export { registerComponent } from './worker'

packages/mizan-ssr/src/test-worker.tsx

@@ -0,0 +1,29 @@
+/**
+ * Test SSR worker — registers simple components for the test suite.
+ */
+
+import { registerComponent } from './worker'
+
+// Simple component that renders props
+function Hello({ name }: { name: string }) {
+    return <div>Hello, {name}!</div>
+}
+
+// Component that renders a list
+function UserProfile({ user_id, name }: { user_id: number; name: string }) {
+    return (
+        <div className="profile">
+            <h1>{name}</h1>
+            <span>ID: {user_id}</span>
+        </div>
+    )
+}
+
+// Component that throws during render
+function Broken() {
+    throw new Error('Intentional render error')
+}
+
+registerComponent('Hello', Hello)
+registerComponent('UserProfile', UserProfile)
+registerComponent('Broken', Broken)

packages/mizan-ssr/src/worker.tsx

@@ -0,0 +1,116 @@
+/**
+ * Mizan SSR Worker — Renders React components to HTML.
+ *
+ * Protocol: newline-delimited JSON-RPC over stdin/stdout.
+ *
+ * Request:  {"id": 1, "method": "render", "params": {"component": "ProfilePage", "props": {...}}}
+ * Response: {"id": 1, "html": "<div>...</div>"}
+ *
+ * Methods:
+ *   render  — Render a registered component to HTML string
+ *   ping    — Health check, returns {"id": N, "pong": true}
+ *
+ * The worker stays alive across requests. Django manages the subprocess.
+ * Components are registered via registerComponent() before the worker starts reading.
+ */
+
+import { renderToString } from 'react-dom/server'
+import { createElement } from 'react'
+import type { ComponentType } from 'react'
+
+const registry = new Map<string, ComponentType<any>>()
+
+/**
+ * Register a React component for SSR rendering.
+ * Call this before the worker starts processing (at module init time).
+ */
+export function registerComponent(name: string, component: ComponentType<any>): void {
+    registry.set(name, component)
+}
+
+interface RenderRequest {
+    id: number
+    method: 'render' | 'ping'
+    params?: {
+        component: string
+        props: Record<string, any>
+    }
+}
+
+interface RenderResponse {
+    id: number
+    html?: string
+    pong?: boolean
+    error?: string
+}
+
+function respond(msg: RenderResponse): void {
+    const line = JSON.stringify(msg) + '\n'
+    Bun.write(Bun.stdout, line)
+}
+
+function handleMessage(msg: RenderRequest): void {
+    if (msg.method === 'ping') {
+        respond({ id: msg.id, pong: true })
+        return
+    }
+
+    if (msg.method === 'render') {
+        const { component, props } = msg.params ?? {}
+
+        if (!component) {
+            respond({ id: msg.id, error: 'Missing component name' })
+            return
+        }
+
+        const Component = registry.get(component)
+        if (!Component) {
+            respond({ id: msg.id, error: `Component '${component}' not registered` })
+            return
+        }
+
+        try {
+            const html = renderToString(createElement(Component, props ?? {}))
+            respond({ id: msg.id, html })
+        } catch (e: any) {
+            respond({ id: msg.id, error: `Render error: ${e.message}` })
+        }
+        return
+    }
+
+    respond({ id: msg.id, error: `Unknown method: ${msg.method}` })
+}
+
+// Read newline-delimited JSON from stdin
+async function processInput(): Promise<void> {
+    const reader = Bun.stdin.stream().getReader()
+    const decoder = new TextDecoder()
+    let buffer = ''
+
+    while (true) {
+        const { done, value } = await reader.read()
+        if (done) break
+
+        buffer += decoder.decode(value, { stream: true })
+
+        let newlineIdx: number
+        while ((newlineIdx = buffer.indexOf('\n')) !== -1) {
+            const line = buffer.slice(0, newlineIdx).trim()
+            buffer = buffer.slice(newlineIdx + 1)
+
+            if (line) {
+                try {
+                    handleMessage(JSON.parse(line))
+                } catch (e: any) {
+                    // Malformed JSON — respond with error if we can parse an id
+                    Bun.write(Bun.stdout, JSON.stringify({ id: -1, error: `Parse error: ${e.message}` }) + '\n')
+                }
+            }
+        }
+    }
+}
+
+// Signal readiness
+Bun.write(Bun.stdout, JSON.stringify({ id: 0, ready: true }) + '\n')
+
+processInput()