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>

backends/mizan-django/tests/models.py

@@ -1,3 +1,5 @@
+import uuid
+
 from django.contrib.auth.models import (
     AbstractBaseUser,
     BaseUserManager,
@@ -7,8 +9,6 @@ from django.db import models
 
 
 class EmailUserManager(BaseUserManager):
-    """Custom user manager using email as the unique identifier."""
-
     def create_user(self, email, password=None, **extra_fields):
         if not email:
             raise ValueError("Email is required")
@@ -25,12 +25,6 @@ class EmailUserManager(BaseUserManager):
 
 
 class EmailUser(AbstractBaseUser, PermissionsMixin):
-    """Minimal user model with email as USERNAME_FIELD.
-
-    Matches the calling convention used in mizan's test suite:
-        User.objects.create_user(email="...", password="...", is_staff=True)
-    """
-
     email = models.EmailField(unique=True)
     is_staff = models.BooleanField(default=False)
     is_active = models.BooleanField(default=True)
@@ -44,11 +38,6 @@ class EmailUser(AbstractBaseUser, PermissionsMixin):
         app_label = "tests"
 
 
-# ─── Shape test models ──────────────────────────────────────────────────────
-
-import uuid
-
-
 class TimestampMixin(models.Model):
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)