A channel's message slots are named from the client, on every backend

The IR called them react-message and django-message, so a FastAPI channel had
to declare a DjangoMessage. They are client-message and server-message now,
and the direction words hold wherever a channel is declared: Params /
ClientMessage / ServerMessage, with mizan-core deriving <Pascal>Params and
friends so no backend names a type itself. Django's ReactChannel and
FastAPI's ReactChannel are both Channel.

mizan-fastapi never registered a channels extension, so build_ir() emitted no
channel at all and every payload type was invisible to codegen. It registers
one now. RegistryExtension is an ABC requiring all(), which is what the IR
reads — an extension that cannot enumerate its registrations no longer exists.

The gate that should have caught the rename could not: tests/afi registered no
channel because mizan-rust had no channel registry to register one in, so a
five-package rename of the wire contract passed byte-parity without a channel
byte crossing it. mizan-rust grows ChannelSlotKind, a CHANNELS slice, a
#[mizan::channel] macro, and KDL emission whose wire_to_pascal matches Python's
split; the AFI fixture now carries a channel with every slot and one with a
single slot, so all three backends prove the contract byte for byte.

MizanChannel held three Option<String> beside three has_*() predicates and
unwrapped them with defaults; it holds an ordered slot vector, so an absent
slot is absent rather than defaulted. The channels target emitted a React
hooks file that a stage1-only consumer could not compile — react emits that
now. The codegen's parity tests byte-compared emitted source against baselines
without ever compiling it: they compile the generated crate and run its tests,
import the generated Python package and call every method, and typecheck each
TypeScript target against a consumer.

Also fixed at source: app_visitor printed its import diagnostic to stdout, the
stream export_mizan_ir writes KDL to, so a failed import silently corrupted the
IR; the apps root was hardcoded to "apps"; _default_literal crashed build_ir on
any non-JSON-serializable field default; Django and mizan-core derived Pascal
names two different ways, disagreeing on every dotted channel name.

ir.py builds a document and renders templates/ir/document.kdl.j2 rather than
appending KDL strings with hand-tracked indentation, and named types resolve to
a fixed point — a model reachable only through a union branch was referenced by
a ref that no type block ever defined.

The rest is the write-gate's own classifiers run over the standing tree:
relative imports, silent swallows, Protocol contracts that should be ABCs,
emitters hand-rendering target source, catch-all arms over closed enums, and
comments narrating the project rather than the code.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/afi/fixture.py

@@ -1,17 +1,10 @@
-"""
-The AFI fixture — a small set of @client-decorated functions designed to
-exercise the protocol axes both backends must agree on:
+"""The @client-decorated functions and the channel wire contracts every AFI
+backend registers.
 
-- 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.
+`register_fixture()` binds the functions into mizan_core.registry. The channel
+classes live in each backend's own app module — the `Channel` base is
+backend-specific — and bind the payload models below under `CHAT_CHANNEL` and
+`USER_ALERTS_CHANNEL`.
 """
 
 from __future__ import annotations
@@ -49,48 +42,82 @@ class StatusOutput(BaseModel):
     ok: bool
 
 
+# ─── Channel wire contracts ─────────────────────────────────────────────────
+
+CHAT_CHANNEL = "chat"
+USER_ALERTS_CHANNEL = "user_alerts"
+
+
+class ChatParams(BaseModel):
+    room_id: str
+
+
+class ChatClientMessage(BaseModel):
+    text: str
+
+
+class ChatServerMessage(BaseModel):
+    text: str
+    from_user: str
+
+
+class UserAlertsServerMessage(BaseModel):
+    body: str
+    unread: int
+
+
+# ─── Stored rows ────────────────────────────────────────────────────────────
+
+_PROFILES = {5: "Ada Lovelace", 6: "Grace Hopper"}
+_ORDERS = [
+    OrderOutput(id=1, user_id=5, total=1200),
+    OrderOutput(id=2, user_id=5, total=350),
+]
+
+
 # ─── 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")
+    # A context member always answers, so an unstored id reads back under a
+    # name derived from the id rather than an absence.
+    return ProfileOutput(
+        user_id=user_id, name=_PROFILES.get(user_id, f"user {user_id}")
+    )
 
 
 @client(context="user")
 def user_orders(request, user_id: int) -> list[OrderOutput]:
-    """Other half of the user context — same param, proves param elevation."""
-    return []
+    return [order for order in _ORDERS if order.user_id == user_id]
 
 
 @client(affects="user")
 def update_profile(request, user_id: int, name: str) -> StatusOutput:
-    """Mutation declaring affects on the user context."""
-    return StatusOutput(ok=True)
+    return StatusOutput(ok=bool(name) and user_id in _PROFILES)
 
 
 @client
 def find_user(request, user_id: int) -> ProfileOutput | None:
-    """Optional return — exercises Pydantic `T | None` schema introspection."""
+    if user_id in _PROFILES:
+        return ProfileOutput(user_id=user_id, name=_PROFILES[user_id])
     return None
 
 
 @client(merge="user")
 def rename_user(request, user_id: int, name: str) -> ProfileOutput:
-    """Merge target — kernel splices return value into the user context."""
+    # The merge target's return value is what the kernel splices into the
+    # `user` context, so the renamed profile is the whole result.
     return ProfileOutput(user_id=user_id, name=name)
 
 
@@ -98,7 +125,6 @@ def rename_user(request, user_id: int, name: str) -> ProfileOutput:
 
 
 def register_fixture() -> None:
-    """Register every fixture function with mizan_core.registry."""
     register(echo, "echo")
     register(whoami, "whoami")
     register(user_profile, "user_profile")