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>

examples/django-react-site/harness/django.config.mjs

@@ -10,9 +10,9 @@ export default {
     source: {
         django: {
             managePath: path.join(root, 'examples/django-react-site/backend/manage.py'),
-            command: [path.join(root, 'packages/mizan-rpc/adapters/django/.venv/bin/python')],
+            command: [path.join(root, 'packages/mizan-django/.venv/bin/python')],
             env: {
-                PYTHONPATH: `${path.join(root, 'packages/mizan-rpc/adapters/django/src')}:${path.join(root, 'examples/django-react-site/backend')}`,
+                PYTHONPATH: `${path.join(root, 'packages/mizan-django/src')}:${path.join(root, 'examples/django-react-site/backend')}`,
                 DJANGO_SETTINGS_MODULE: 'testapp.settings',
             },
         },