Extract client/function.py to mizan-core (Tier B)

The @client decorator + ServerFunction base + composition machinery is
mostly framework-agnostic. The only Django couplings were typing
(HttpRequest in __init__ and submit_handler signatures) and runtime
view-path detection (HttpResponseBase isinstance/issubclass checks).

Replaced both with backend-extension hooks:

- HttpRequest type hints → Any. Type Protocol can be tightened later.
- HttpResponseBase view-path detection → set_framework_response_base(cls)
  hook in mizan_core.client.function. Backends register their framework's
  response base at import time. is_framework_response(obj_or_cls) handles
  both instance and subclass checks via the registered base.

mizan-django registers HttpResponseBase via mizan/client/__init__.py
before any @client-decorated code is loaded. FastAPI would similarly
register starlette.responses.Response.

Direct consumers updated:
- mizan/setup/discovery.py: ServerFunction import path
- mizan/forms/__init__.py: ServerFunction + create_form_functions imports

mizan/client/__init__.py keeps its public re-export surface stable so
'from mizan.client import client, ServerFunction, …' continues to work
for downstream Django consumers.

Verified:
- mizan-core: 15/15
- mizan-django: 348 pass, 21 skip, 0 fail
- mizan-ts edge-compat: 34/34

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

backends/mizan-django/src/mizan/client/__init__.py

@@ -2,16 +2,24 @@
 mizan.client - Server function implementation.
 
 This subpackage contains everything needed to make server functions work:
-- The @client decorator
-- ServerFunction base class
-- Function execution logic
-- JWT authentication (integral to server functions)
+- The @client decorator (lives in mizan_core.client.function)
+- ServerFunction base class (mizan_core.client.function)
+- Function execution logic (.executor — Django-specific dispatch)
+- JWT authentication (.jwt — Django-specific session integration)
 
 Usage:
     from mizan.client import client, ServerFunction, compose
 """
 
-from .function import (
+# Register the Django framework response base so view-path detection works
+# in mizan_core.client.function. Has to happen before any @client-decorated
+# code is evaluated.
+from django.http import HttpResponseBase as _HttpResponseBase
+from mizan_core.client.function import set_framework_response_base as _set_response_base
+_set_response_base(_HttpResponseBase)
+
+
+from mizan_core.client.function import (
     # Decorator
     client,
     # Context markers

backends/mizan-django/src/mizan/forms/__init__.py

@@ -293,7 +293,7 @@ def _register_form_as_server_functions(form_class: type) -> None:
     from .schema_utils import build_form_schema
     from .validation_utils import validate_form_instance
     from mizan_core.registry import register
-    from mizan.client.function import ServerFunction
+    from mizan_core.client.function import ServerFunction
 
     config: mizanFormMeta = form_class.mizan
     form_name = config.name
@@ -485,7 +485,7 @@ def _register_formset_functions(
     from .validation_utils import build_formset_validation
     from .formset_utils import forms_to_formset_post_data
     from mizan_core.registry import register
-    from mizan.client.function import ServerFunction
+    from mizan_core.client.function import ServerFunction
 
     formset_class = formset_factory(form_class)
 
@@ -646,7 +646,7 @@ def register_form(
     Creates and registers `{name}.schema`, `{name}.validate`, and
     `{name}.submit` (if a submit_handler is provided).
     """
-    from mizan.client.function import create_form_functions
+    from mizan_core.client.function import create_form_functions
     from mizan_core.registry import register
 
     schema_fn, validate_fn, submit_fn = create_form_functions(

backends/mizan-django/src/mizan/setup/discovery.py

@@ -19,7 +19,7 @@ from typing import Any
 from mizan._vendor.app_visitor import DjangoAppVisitor, get_members
 
 from mizan_core.registry import register, get_function
-from mizan.client.function import ServerFunction
+from mizan_core.client.function import ServerFunction
 
 
 class _RegisterServerFunctions:

cores/mizan-python/src/mizan_core/client/function.py

@@ -35,10 +35,37 @@ from typing import (
     get_type_hints,
 )
 
-from django.http import HttpRequest
 from pydantic import BaseModel
 
 
+# ─── Framework-response-base hook ───────────────────────────────────────────
+#
+# View-path detection — distinguishing functions that return data (RPC path)
+# from functions that return a framework-native response object (view path) —
+# requires knowing the framework's response base class. Each backend adapter
+# registers its base class here at import time.
+#
+# Django sets this to django.http.HttpResponseBase. FastAPI would set it to
+# starlette.responses.Response. If unset, all functions are treated as RPC.
+
+_framework_response_base: type | None = None
+
+
+def set_framework_response_base(cls: type) -> None:
+    """Backends register their framework's response base class for view-path detection."""
+    global _framework_response_base
+    _framework_response_base = cls
+
+
+def is_framework_response(obj_or_cls: Any) -> bool:
+    """True if obj_or_cls is, or is a subclass of, the registered framework response base."""
+    if _framework_response_base is None:
+        return False
+    if isinstance(obj_or_cls, type):
+        return issubclass(obj_or_cls, _framework_response_base)
+    return isinstance(obj_or_cls, _framework_response_base)
+
+
 # =============================================================================
 # REACT CONTEXT - Named context marker
 # =============================================================================
@@ -116,8 +143,8 @@ class ServerFunction(ABC, Generic[TInput, TOutput]):
     Input: ClassVar[type[BaseModel]] = BaseModel
     Output: ClassVar[type[BaseModel]] = BaseModel
 
-    def __init__(self, request: HttpRequest):
-        """Initialize with the Django request."""
+    def __init__(self, request: Any):
+        """Initialize with the framework's request object (HttpRequest in Django, Request in FastAPI, etc.)."""
         self.request = request
 
     @property
@@ -187,9 +214,8 @@ class _FunctionWrapper(ServerFunction):
         else:
             result = self._wrapped_fn(self.request)
 
-        # View path — return HttpResponse directly (no serialization)
-        from django.http import HttpResponseBase
-        if isinstance(result, HttpResponseBase):
+        # View path — return a framework-native response directly (no serialization)
+        if is_framework_response(result):
             return result
 
         # Wrap primitive returns in the generated output model
@@ -431,12 +457,10 @@ def _create_server_function(
     if output_type is None:
         raise TypeError(f"Server function '{name}' must have a return type annotation")
 
-    # Detect view path: function returns HttpResponse (or has no return annotation
-    # that maps to a model — view functions often just have -> HttpResponse)
-    from django.http import HttpResponseBase
-    is_view_path = (
-        isinstance(output_type, type) and issubclass(output_type, HttpResponseBase)
-    )
+    # Detect view path: function returns a framework-native response type
+    # (e.g. Django HttpResponse, FastAPI Response). View functions often just
+    # have -> HttpResponse with no Pydantic model.
+    is_view_path = is_framework_response(output_type)
 
     if is_view_path:
         # View path — no Pydantic output wrapping needed
@@ -776,7 +800,7 @@ class FormSchemaOutput(BaseModel):
 def create_form_functions(
     form_class: type,
     name: str,
-    submit_handler: Callable[[HttpRequest, dict], BaseModel] | None = None,
+    submit_handler: Callable[[Any, dict], BaseModel] | None = None,
 ) -> tuple[type[ServerFunction], type[ServerFunction], type[ServerFunction] | None]:
     """
     Generate server functions for a Django Form.