Build mizan-fastapi MVP — HTTP RPC + context bundling

The Blazr-critical surface for FastAPI. Forms, Channels, Shapes, SSR,
and MWT are out of scope (Ryth's call: defer until Blazr exercises
them; FastAPI projects use native equivalents anyway).

What ships:
- POST /api/mizan/call/      RPC dispatch with Pydantic input validation
- GET  /api/mizan/ctx/{name}/ bundled context fetch (all functions in
                              the named context, parallel-evaluated, single
                              JSON response)
- JSON-body invalidation transport (the 'invalidate' field on mutation
  responses, with auto-scoping when mutation arg names match context params)
- Auth check infrastructure expecting request.state.user populated by
  FastAPI middleware/deps (matches FastAPI idioms)
- Cache-Control: no-store on all responses

Built on existing mizan-core: registry (function lookup, context groups,
invalidation metadata), client.function (the @client decorator + ServerFunction
+ _FunctionWrapper). No code copied or duplicated from mizan-django — the
shared substrate is genuinely shared.

Package layout:
  backends/mizan-fastapi/
    pyproject.toml         distribution=mizan-fastapi, module=mizan_fastapi
    src/mizan_fastapi/
      executor.py          dispatch + auth + invalidation
      router.py            FastAPI APIRouter with the two endpoints
    tests/test_dispatch.py 11 e2e tests against TestClient

Test fixture establishes the registration pattern: explicit
register(fn_class, "name") after each @client. mizan-fastapi doesn't
ship discovery — apps register their functions explicitly. (mizan-django
keeps its DjangoAppVisitor discovery; FastAPI's lack of an app system
makes auto-discovery less natural.)

Makefile: install + test targets now include mizan-fastapi alongside
the other packages. New test-core / test-fastapi targets added for
symmetry.

Verified:
- mizan-core: 15/15
- mizan-django: 348 pass, 21 skip, 0 fail
- mizan-fastapi: 11/11
- mizan-ts edge-compat: 34/34

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

Makefile

@@ -1,22 +1,31 @@
-.PHONY: install test test-django test-react test-integration docker-up docker-down clean
+.PHONY: install test test-core test-django test-fastapi test-react test-integration docker-up docker-down clean
 
+CORE = cores/mizan-python
 DJANGO = backends/mizan-django
+FASTAPI = backends/mizan-fastapi
 REACT = frontends/mizan-react
 
 # ─── Setup ───────────────────────────────────────────────────────────────────
 
 install:
-	cd cores/mizan-python && uv pip install -e .
+	cd $(CORE) && uv pip install -e .
 	cd $(DJANGO) && uv pip install -e ".[dev,channels]"
+	cd $(FASTAPI) && uv pip install -e ".[dev]"
 	cd $(REACT) && npm install
 
 # ─── Unit Tests ──────────────────────────────────────────────────────────────
 
-test: test-django test-react
+test: test-core test-django test-fastapi test-react
+
+test-core:
+	cd $(CORE) && uv run --extra dev pytest
 
 test-django:
 	cd $(DJANGO) && uv run pytest
 
+test-fastapi:
+	cd $(FASTAPI) && uv run pytest
+
 test-react:
 	cd $(REACT) && npm test