Flatten to three packages + extract mizan-runtime

packages/
  mizan-runtime/   Framework-agnostic state engine (~150 lines)
                   Context registry, batched invalidation, fetch primitives
  mizan-django/    Django server adapter (was packages/mizan-rpc/adapters/django/)
                   Codegen moved to mizan-django/generate/
  mizan-react/     React adapter (was packages/mizan-csr/adapters/react/)

Removed premature abstractions: mizan-ast, mizan-schema, mizan-rpc,
mizan-csr, mizan-ssr stub packages. The actual architecture is three
concrete packages, not five abstract layers.

mizan-runtime implements the v1 spec: registerContext with params,
scoped invalidation via microtask batching, server-driven invalidation
from mutation responses, mizanFetch for context bundles, mizanCall for
mutations.

264 Django + 33 React tests pass.

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

packages/mizan-react/src/client/RouterContext.tsx

@@ -0,0 +1,43 @@
+'use client'
+
+import { createContext, useContext, type ReactNode } from 'react'
+
+/**
+ * Framework-agnostic router adapter.
+ * Implement this interface for your framework (Next.js, Remix, etc.)
+ */
+export interface RouterAdapter {
+    /** Navigate to a path (adds to history) */
+    push: (path: string) => void
+    /** Replace current path (no history entry) */
+    replace: (path: string) => void
+    /** Current pathname (e.g., "/account/login") */
+    pathname: string
+    /** Current search params */
+    searchParams: URLSearchParams
+    /** Get a specific route param (e.g., from /auth/[...path]) - optional */
+    getParam?: (name: string) => string | string[] | undefined
+}
+
+const Context = createContext<RouterAdapter | null>(null)
+
+interface RouterContextProps {
+    children: ReactNode
+    router: RouterAdapter
+}
+
+/**
+ * Provides router adapter to route guards.
+ */
+export function RouterContext({ children, router }: RouterContextProps) {
+    return <Context value={router}>{children}</Context>
+}
+
+/**
+ * Hook to access router adapter.
+ */
+export function useRouter(): RouterAdapter {
+    const ctx = useContext(Context)
+    if (!ctx) throw new Error('useRouter must be used within RouterContext')
+    return ctx
+}