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/django_app/afi_app/apps.py

@@ -7,7 +7,8 @@ class AfiAppConfig(AppConfig):
     name = "afi_app"
 
     def ready(self) -> None:
-        # tests/afi/ is on sys.path (added by manage.py), so `fixture` resolves
-        # to tests/afi/fixture.py — the same module the FastAPI side imports.
+        # manage.py puts tests/afi/ on sys.path, so `fixture` resolves there.
         from fixture import register_fixture
+        from afi_app.channels import register_fixture_channels
         register_fixture()
+        register_fixture_channels()

tests/afi/django_app/afi_app/channels.py

@@ -0,0 +1,44 @@
+"""The AFI fixture's two channels bound to Django's `Channel` base.
+
+Payload models and wire names come from `fixture`; `authorize` and `group`
+are the obligations the Django base adds.
+"""
+
+from mizan.channels import Channel, register
+
+from fixture import (
+    CHAT_CHANNEL,
+    USER_ALERTS_CHANNEL,
+    ChatClientMessage,
+    ChatParams,
+    ChatServerMessage,
+    UserAlertsServerMessage,
+)
+
+
+class ChatChannel(Channel):
+    Params = ChatParams
+    ClientMessage = ChatClientMessage
+    ServerMessage = ChatServerMessage
+
+    def authorize(self, params: ChatParams | None = None) -> bool:
+        return True
+
+    def group(self, params: ChatParams | None = None) -> str:
+        # Params are absent on the schema-export path, where no room is bound.
+        return f"chat:{params.room_id}" if params else "chat"
+
+
+class UserAlertsChannel(Channel):
+    ServerMessage = UserAlertsServerMessage
+
+    def authorize(self, params: None = None) -> bool:
+        return True
+
+    def group(self, params: None = None) -> str:
+        return "user_alerts"
+
+
+def register_fixture_channels() -> None:
+    register(ChatChannel, CHAT_CHANNEL)
+    register(UserAlertsChannel, USER_ALERTS_CHANNEL)

tests/afi/django_app/project/urls.py

@@ -1,3 +1 @@
-"""Empty URLconf — the AFI fixture only exercises the schema export."""
-
 urlpatterns: list = []