React wrapper-layer codegen — restore the idioms over the kernel

The harness was written against the MIZAN.md oracle (<MizanContext>,
provider-per-context, useMizan, etc.) but the codegen had been narrowed
to just hooks-direct-on-kernel after the kernel split. Restoring the
React-idiomatic layer on top of the kernel.

backends/mizan-django/generate/generator/lib/adapters/react.mjs:
- Emits <MizanContext baseUrl="…"> root provider that calls configure()
  once and (if a global context is registered) wraps children in
  <GlobalContextProvider>.
- Emits <GlobalContextProvider> + <{Name}Context> per named context —
  kernel registration happens once per provider mount, not per hook
  call. Consumers read from React Context.
- Base hooks: useGlobalContext() / use{Name}Context() return full
  ContextState<T> (data + status + error).
- Convenience hooks per context-function (use{Fn}() returns data | null)
  and per regular function/mutation (use{Fn}() returns
  { mutate, isPending, error }).
- useMizan() returns { call, fetch } as an imperative escape hatch
  for test harnesses or rare cases where typed hooks don't fit.
- Re-exports MizanError, configure, initSession, ContextState from
  @mizan/base.

backends/mizan-django/generate/generator/cli.mjs:
- After Stage 2, appends `export * from './<adapter>'` to index.ts so
  `import { useEcho, MizanContext } from './api'` works as a barrel.

Bug fixes surfaced during integration:
- react.mjs was generating `from '../index'` (wrong path); flat layout
  needs `./index`.
- harness django.config.mjs had `output: 'src/api/generated.ts'` which
  the codegen treated as a directory; corrected to `output: 'src/api'`.
- example testapp/clients.py imported from the deleted
  mizan.setup.registry path; routed through mizan.setup aggregator.

harness/package.json: adds @mizan/base dep so the generated react.tsx
can resolve its kernel imports.

harness/src/fixtures.tsx:
- DjangoError → MizanError (kernel error class, backend-agnostic).
- useChatChannel sourced from ./api/channels.hooks directly (not
  re-exported from the unified index for now).
- Form fixtures removed — forms codegen deferred per Blazr scope.

Verified: harness `vite build` succeeds, 53 modules transformed.

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

examples/django-react-site/harness/src/fixtures.tsx

@@ -23,11 +23,10 @@ import {
     useBuggyFn,
     usePermissionCheckFn,
     useCurrentUser,
-    DjangoError,
+    MizanError,
     useMizan,
-    useChatChannel,
 } from './api'
-import { useContactForm, useLoginForm } from './api/generated.forms'
+import { useChatChannel } from './api/channels.hooks'
 
 // ─── Fixture router ─────────────────────────────────────────────────────────
 
@@ -55,9 +54,7 @@ export function Fixtures() {
         case 'permission-error': return <PermissionError_ />
         case 'permission-success': return <PermissionSuccess />
         case 'context-current-user': return <ContextCurrentUser />
-        case 'form-login-schema': return <FormLoginSchema />
-        case 'form-contact-schema': return <FormContactSchema />
-        case 'form-contact-submit': return <FormContactSubmit />
+        // Form fixtures removed — forms codegen deferred per Blazr scope
         case 'channel-chat': return <ChannelChatFixture />
         default: return <div data-testid="ready">Harness ready. Set #hash.</div>
     }
@@ -74,10 +71,10 @@ function Result({ data, error }: { data?: unknown; error?: unknown }) {
             {error !== undefined && error !== null && (
                 <>
                     <div data-testid="error-type">
-                        {error instanceof DjangoError ? 'DjangoError' : 'Error'}
+                        {error instanceof MizanError ? 'MizanError' : 'Error'}
                     </div>
                     <div data-testid="error-code">
-                        {error instanceof DjangoError ? error.code : ''}
+                        {error instanceof MizanError ? error.code : ''}
                     </div>
                     <pre data-testid="error-message">
                         {error instanceof Error ? error.message : String(error)}
@@ -187,44 +184,6 @@ function ContextCurrentUser() {
     }
 }
 
-// ─── Form fixtures (using generated form hooks) ─────────────────────────────
-
-function FormLoginSchema() {
-    const form = useLoginForm()
-    if (form.loading) return <div>loading...</div>
-    return <pre data-testid="result">{JSON.stringify(form.schema)}</pre>
-}
-
-function FormContactSchema() {
-    const form = useContactForm()
-    if (form.loading) return <div>loading...</div>
-    return <pre data-testid="result">{JSON.stringify(form.schema)}</pre>
-}
-
-function FormContactSubmit() {
-    const form = useContactForm()
-    const [result, setResult] = useState<unknown>()
-    const [submitted, setSubmitted] = useState(false)
-
-    useEffect(() => {
-        if (!form.loading && !submitted) {
-            form.set('name', 'Test User')
-            form.set('email', 'test@example.com')
-            form.set('message', 'Hello from e2e')
-            setSubmitted(true)
-        }
-    }, [form.loading, submitted, form])
-
-    useEffect(() => {
-        if (submitted && !result) {
-            form.submit().then(setResult)
-        }
-    }, [submitted, result, form])
-
-    if (!result) return <div>loading...</div>
-    return <pre data-testid="result">{JSON.stringify(result)}</pre>
-}
-
 // ─── Channel fixtures ───────────────────────────────────────────────────────
 
 function ChannelChatFixture() {