Mizan codegen substrate: Rust kernel + Rust codegen binary, JS generator deleted

The Mizan codegen substrate moves off JavaScript template-literal emission
onto a compiled Rust binary that consumes the same OpenAPI + x-mizan-* IR
the JS substrate consumed. Three structural wins fall out of one move:

1. Moat closes. The codegen logic (how `affects` becomes auto-invalidation,
   how named contexts collapse onto bundled fetches, how the registry-to-
   Provider mapping is shaped) ships compiled instead of as source bytes
   in every consumer's node_modules.

2. Pattern F (lines.push append-walls) becomes structurally unauthorable.
   The emit substrate is askama templates in templates/<target>/*.j2 —
   actual target-language files with {{ ... }} substitution markers,
   syntax-highlighted natively, type-checked against the render context
   structs at compile time. The Rust emit modules build typed render
   contexts and call .render(); no string-builder surface exists.

3. OpenAPI `default`-bearing fields now emit as non-optional in TS / Python
   / Rust — the server always populates them, so consumer code reads them
   without nullable checks. Surfaced by Blazr's typecheck on regeneration.

Layout:
  frontends/mizan-rust/        — Rust port of @mizan/base; #[cfg(feature="pyo3")]
                                 exposes PyMizanClient for the Python target.
  protocol/mizan-codegen/      — codegen binary source + askama templates.
  protocol/mizan-generate/     — npm-package shim. bin/launcher.mjs dispatches
                                 to the platform-appropriate prebuilt binary.
                                 Old generator/ JS tree deleted.
  tests/rust/                  — wire-parity drivers. drive_kernel exercises
                                 raw client.call() / fetch_context(); drive_emitted
                                 exercises the typed crate the codegen emits.
  tests/afi/afi_codegen_app.py — codegen entrypoint module (imports + registers).
  backends/mizan-fastapi/.../schema.py — adds outputNullable so the Rust
                                 codegen can wrap T | None responses in Option<T>.

Verification:
  - 20 mizan-codegen tests green (IR deserialization, byte-equivalent
    parity vs JS baseline for stage1/rust/python/react/vue/svelte,
    structural test for channels).
  - tests/rust/run_wire_parity.py — 12/12 probes green via the Rust binary
    driving the FastAPI fixture end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

protocol/mizan-codegen/tests/fixtures/js_react/react.tsx

@@ -0,0 +1,157 @@
+'use client'
+
+// AUTO-GENERATED by mizan — do not edit
+
+import {
+    createContext,
+    useCallback,
+    useContext,
+    useEffect,
+    useRef,
+    useState,
+    useSyncExternalStore,
+    type ReactNode,
+} from 'react'
+import {
+    configure,
+    initSession,
+    mizanCall,
+    mizanFetch,
+    MizanError,
+    registerContext,
+    type ContextState,
+} from '@mizan/base'
+
+import { fetchUserContext, type UserContextData, type UserContextParams, callUpdateProfile, callEcho, callWhoami, callFindUser, callRenameUser, type userProfileOutput, type userOrdersOutput } from './index'
+
+// Internal — runs inside a Provider, registers with the kernel exactly once.
+function useContextSubscription<T>(
+    name: string,
+    params: Record<string, any>,
+    fetchFn: () => Promise<T>,
+    initialData?: T,
+): ContextState<T> {
+    const ref = useRef<ReturnType<typeof registerContext> | null>(null)
+    if (!ref.current) {
+        ref.current = registerContext(name, params, fetchFn, initialData)
+    }
+    const handle = ref.current
+
+    useEffect(() => {
+        if (handle.getState().status === 'idle') handle.refetch()
+        return () => handle.unregister()
+    }, [handle])
+
+    return useSyncExternalStore(handle.subscribe, handle.getState, handle.getState)
+}
+
+// Internal — wraps an imperative call() with isPending / error state.
+interface MutationHook<TArgs, TResult> {
+    mutate: (args: TArgs) => Promise<TResult>
+    isPending: boolean
+    error: Error | null
+}
+
+function useMutation<TArgs, TResult>(
+    callFn: (args: TArgs) => Promise<TResult>,
+): MutationHook<TArgs, TResult> {
+    const [isPending, setIsPending] = useState(false)
+    const [error, setError] = useState<Error | null>(null)
+
+    const mutate = useCallback(async (args: TArgs) => {
+        setIsPending(true)
+        setError(null)
+        try {
+            return await callFn(args)
+        } catch (e) {
+            setError(e as Error)
+            throw e
+        } finally {
+            setIsPending(false)
+        }
+    }, [callFn])
+
+    return { mutate, isPending, error }
+}
+
+// ── User Context ──
+
+const UserCtx = createContext<ContextState<UserContextData> | null>(null)
+
+export function UserContext({ children, ...params }: UserContextParams & { children: ReactNode }) {
+    const state = useContextSubscription('user', params, () => fetchUserContext(params))
+    return <UserCtx.Provider value={state}>{children}</UserCtx.Provider>
+}
+
+export function useUserContext(): ContextState<UserContextData> {
+    const ctx = useContext(UserCtx)
+    if (!ctx) throw new Error('useUserContext requires <UserContext>')
+    return ctx
+}
+
+export function useUserProfile(): userProfileOutput | null {
+    return useUserContext().data?.user_profile ?? null
+}
+
+export function useUserOrders(): userOrdersOutput | null {
+    return useUserContext().data?.user_orders ?? null
+}
+
+export function useUpdateProfile() {
+    return useMutation<Parameters<typeof callUpdateProfile>[0], Awaited<ReturnType<typeof callUpdateProfile>>>(callUpdateProfile)
+}
+
+export function useEcho() {
+    return useMutation<Parameters<typeof callEcho>[0], Awaited<ReturnType<typeof callEcho>>>(callEcho)
+}
+
+export function useWhoami() {
+    return useMutation<void, Awaited<ReturnType<typeof callWhoami>>>(() => callWhoami() as any)
+}
+
+export function useFindUser() {
+    return useMutation<Parameters<typeof callFindUser>[0], Awaited<ReturnType<typeof callFindUser>>>(callFindUser)
+}
+
+export function useRenameUser() {
+    return useMutation<Parameters<typeof callRenameUser>[0], Awaited<ReturnType<typeof callRenameUser>>>(callRenameUser)
+}
+
+// ── MizanContext root provider ──
+
+export interface MizanContextProps {
+    /** Base URL for protocol endpoints. Defaults to "/api/mizan". */
+    baseUrl?: string
+    /** Set to `false` for backends without a `/session/` endpoint (e.g. FastAPI). */
+    session?: boolean
+    children: ReactNode
+}
+
+/**
+ * Root provider — calls configure() once and mounts the global context (if defined).
+ * Must wrap any component using Mizan-generated hooks.
+ */
+export function MizanContext({ baseUrl, session, children }: MizanContextProps) {
+    const configured = useRef(false)
+    if (!configured.current) {
+        const opts: Parameters<typeof configure>[0] = {}
+        if (baseUrl !== undefined) opts.baseUrl = baseUrl
+        if (session !== undefined) opts.session = session
+        if (Object.keys(opts).length > 0) configure(opts)
+        configured.current = true
+    }
+    return <>{children}</>
+}
+
+// ── Imperative escape hatch ──
+
+/**
+ * Returns the imperative kernel API. For test harnesses or rare cases where
+ * a typed generated hook does not fit. Most app code should use the typed hooks.
+ */
+export function useMizan() {
+    return { call: mizanCall, fetch: mizanFetch }
+}
+
+export type { ContextState } from '@mizan/base'
+export { configure, initSession, MizanError } from '@mizan/base'