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>

protocol/mizan-codegen/templates/python/client.py.j2

@@ -1,5 +1,3 @@
-# AUTO-GENERATED by mizan — do not edit
-
 from __future__ import annotations
 
 from collections.abc import Callable
@@ -25,8 +23,19 @@ class MizanClient:
             csrf_header_name=csrf_header_name,
         )
 
-{{ ctx_methods_block }}
-{{ call_methods_block }}
+{% for ctx in contexts %}    def fetch_{{ ctx.snake }}_context(self{% for p in ctx.params %}, {{ p.ident }}: {{ p.ty }}{% if !p.required %} | None = None{% endif %}{% endfor %}) -> "{{ ctx.data_class }}":
+        raw = self._inner.fetch_context("{{ ctx.name }}", {{ "{" }}{% for p in ctx.params %}{% if !loop.first %}, {% endif %}"{{ p.raw_name }}": {{ p.ident }}{% endfor %}{{ "}" }})
+        return {{ ctx.data_class }}(**raw)
+    def subscribe_{{ ctx.snake }}_context(self{% for p in ctx.params %}, {{ p.ident }}: {{ p.ty }}{% if !p.required %} | None = None{% endif %}{% endfor %},
+                                       callback: Callable[[dict[str, Any]], None]) -> PyContextSubscription:
+        return self._inner.subscribe_context("{{ ctx.name }}", {{ "{" }}{% for p in ctx.params %}{% if !loop.first %}, {% endif %}"{{ p.raw_name }}": {{ p.ident }}{% endfor %}{{ "}" }}, callback)
+{% if !loop.last %}
+{% endif %}{% endfor %}
+{% for call in calls %}    def call_{{ call.snake }}(self{% match call.input %}{% when CallInput::Typed with (t) %}, args: {{ crate::emit::casing::pascal_case(t) }}{% when CallInput::Absent %}{% endmatch %}) -> {{ call.output }}{% if call.nullable %} | None{% endif %}:
+        raw = self._inner.call("{{ call.wire_name }}", {% match call.input %}{% when CallInput::Typed with (t) %}args.model_dump(){% when CallInput::Absent %}{}{% endmatch %})
+        return {{ call.output }}(**raw){% if call.nullable %} if raw is not None else None{% endif %}
+{% if !loop.last %}
+{% endif %}{% endfor %}
     def invalidate(self, context: str) -> None:
         self._inner.invalidate(context)
 
@@ -34,6 +43,8 @@ class MizanClient:
         self._inner.invalidate_scoped(context, params)
 
 
-# ── Context data shapes (per-context bundle) ──────────────────────────────
-
-{{ data_classes_block }}
+{% for dc in data_classes %}class {{ dc.class_name }}(BaseModel):
+    """Bundled return of fetch_{{ dc.snake }}_context."""
+{% for f in dc.fields %}    {{ f.ident }}: {{ f.ty }}{% if f.nullable %} | None{% endif %}
+{% endfor %}{% if !loop.last %}
+{% endif %}{% endfor %}