AFI conformance test suite

Substrate-level gate: same @client fixture registered in both backends
emits equivalent schemas, therefore the codegen produces equivalent
TypeScript regardless of which backend the frontend is generated against.
Catches adapter symmetry problems (Pydantic→OpenAPI converter divergence,
metadata leakage, ordering non-determinism) without docker, browser, or
Playwright.

What ships:

backends/mizan-fastapi/src/mizan_fastapi/schema.py — build_schema():
- Builds OpenAPI 3.0 from registered Mizan functions, mirroring the
  shape mizan-django's export emits.
- Drives FastAPI's native OpenAPI generation by registering a stub POST
  endpoint per function with its Input/Output Pydantic models, then
  appends x-mizan-functions and x-mizan-contexts extensions.
- Param-elevation logic mirrors mizan-django/src/mizan/export/__init__.py
  exactly (sharedBy tracking, required iff every function in context has
  the param).
- snake_to_camel and metadata field shapes match Django for byte-equality
  on the AFI surface.

tests/afi/ — the conformance harness:
- fixture.py: 5 @client functions covering the protocol axes (plain,
  context, mutation+affects). No channels/forms — those aren't AFI-common.
- django_app/: minimal Django project (settings, urls, AppConfig.ready
  registers the fixture). manage.py adds tests/afi/ to sys.path so both
  backends import the same fixture module.
- fastapi_app.py: thin make_app() that registers fixture and mounts router.
- schema_normalizer.py: drops backend-specific framing — Ninja-vs-FastAPI
  envelope differences (info/servers/tags), Django-only function fields
  (form metadata), x-mizan-channels. Plus afi_subset() and
  function_io_schemas() helpers for narrower comparisons.
- test_codegen_parity.py: three gates
  1. x-mizan-functions match across backends
  2. x-mizan-contexts match across backends
  3. Per-function Input/Output OpenAPI schemas match (what codegen feeds
     to openapi-typescript for type generation)

The full normalized OpenAPI envelopes do diverge — FastAPI adds
HTTPValidationError, the two converters wrap things slightly differently
in non-AFI-essential ways. That's not in the test scope. The codegen
only consumes x-mizan-functions, x-mizan-contexts, and the per-function
type schemas; those are what the test gates.

Makefile: test-afi target added; rolls into the test aggregate.

Verified: 3/3 conformance tests pass. Other surfaces unaffected —
mizan-core 15/15, mizan-django 348 pass, mizan-fastapi 11/11.

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

tests/afi/fixture.py

@@ -0,0 +1,94 @@
+"""
+The AFI fixture — a small set of @client-decorated functions designed to
+exercise the protocol axes both backends must agree on:
+
+- plain function with typed input
+- plain function with no input
+- two context functions sharing a param (proves bundling + param elevation)
+- a mutation declaring `affects` on the context
+
+No channels, no forms, no shapes — those aren't AFI-common.
+
+`register_fixture()` registers the functions with mizan_core.registry.
+Backend test apps import this module and call register_fixture() during
+their setup so each backend's schema export sees the same registrations.
+"""
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+from mizan_core.client.function import client
+from mizan_core.registry import register
+
+
+# ─── Output shapes ──────────────────────────────────────────────────────────
+
+
+class EchoOutput(BaseModel):
+    message: str
+
+
+class WhoamiOutput(BaseModel):
+    email: str
+    authenticated: bool
+
+
+class ProfileOutput(BaseModel):
+    user_id: int
+    name: str
+
+
+class OrderOutput(BaseModel):
+    id: int
+    user_id: int
+    total: int
+
+
+class StatusOutput(BaseModel):
+    ok: bool
+
+
+# ─── Fixture functions ──────────────────────────────────────────────────────
+
+
+@client
+def echo(request, text: str) -> EchoOutput:
+    """Echoes the input back."""
+    return EchoOutput(message=f"echo: {text}")
+
+
+@client
+def whoami(request) -> WhoamiOutput:
+    """Returns the current user identity."""
+    return WhoamiOutput(email="anon@example.com", authenticated=False)
+
+
+@client(context="user")
+def user_profile(request, user_id: int) -> ProfileOutput:
+    """One half of the user context."""
+    return ProfileOutput(user_id=user_id, name="placeholder")
+
+
+@client(context="user")
+def user_orders(request, user_id: int) -> list[OrderOutput]:
+    """Other half of the user context — same param, proves param elevation."""
+    return []
+
+
+@client(affects="user")
+def update_profile(request, user_id: int, name: str) -> StatusOutput:
+    """Mutation declaring affects on the user context."""
+    return StatusOutput(ok=True)
+
+
+# ─── Registration ───────────────────────────────────────────────────────────
+
+
+def register_fixture() -> None:
+    """Register every fixture function with mizan_core.registry."""
+    register(echo, "echo")
+    register(whoami, "whoami")
+    register(user_profile, "user_profile")
+    register(user_orders, "user_orders")
+    register(update_profile, "update_profile")