Split the registry — function/composition core + backend extensions

The original registry tangled function, channel, composition, and form
registration in a single file with polymorphic register() dispatch.
That predates the household discipline; it was the design that was
supposed to ship but didn't. Re-implementing the original intent.

cores/mizan-python/src/mizan_core/registry.py (new):
- _functions, _compositions dicts
- register() — ServerFunction-only, no polymorphic dispatch
- register_as(), register_compose()
- register_extension(name, extension) — hook interface
- get_function/get_compose/get_all_functions/get_all_compositions
- get_contexts, get_context_groups
- get_registry, get_schema — aggregate extension contributions
- validate_registry, clear_registry — cascade-clear extensions

RegistryExtension Protocol:
- schema() returns the extension's schema subdict (keyed under its name)
- clear() resets extension state (called by clear_registry)

mizan-django/src/mizan/channels/__init__.py:
- _ChannelsExtension wraps the channel _registry, plugs into core via
  register_extension('channels', ...). Schema output preserves the
  same shape codegen consumed before (snake_case keys, type+bidirectional).

mizan-django/src/mizan/forms/__init__.py:
- register_form() and get_forms() helpers moved here (were in setup/registry.py)
- Both use mizan_core.registry under the hood. Forms don't need a
  separate extension because form sub-functions register as regular
  ServerFunctions with meta.form set.

mizan-django/src/mizan/setup/registry.py: deleted.
mizan-django/src/mizan/setup/__init__.py: re-exports the registry helpers
from mizan_core.registry / mizan.channels / mizan.forms — the Django
adapter's curated public API surface stays stable for users.

Consumers updated: ~10 files imported `from mizan.setup.registry`;
all switched to direct imports from mizan_core.registry, mizan.channels,
or mizan.forms as appropriate. ChannelTests in test_core.py rewritten
to use mizan.channels.register directly (no more polymorphic
@register_as on ReactChannel subclasses).

Verified:
- mizan-core: 15/15
- mizan-django: 348 pass, 21 skip, 0 fail
- mizan-ts edge-compat: 34/34 (cross-language pin holds)

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

backends/mizan-django/README.md

@@ -26,7 +26,7 @@ application = wrap_asgi(get_asgi_application())
 
 ```python
 from mizan.client import client
-from mizan.setup.registry import register
+from mizan_core.registry import register
 from pydantic import BaseModel
 
 class Output(BaseModel):