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/channels/channels.hooks.tsx.j2

@@ -1,26 +1,20 @@
 'use client'
 
-// AUTO-GENERATED by mizan — do not edit
-
 import { useChannel, type ChannelSubscription } from 'mizan/channels'
 
 {% if !type_imports.is_empty() -%}
 import type { {{ type_imports|join(", ") }} } from './channels'
 
 {% endif -%}
-// ── Channel Hooks ─────────────────────────────────────────────────────────
-
-{% for ch in channels -%}
-/**
- * Hook for the {{ ch.name }} channel.
- */
-{% if ch.has_params -%}
-export function use{{ ch.pascal_name }}Channel(params: {{ ch.params_type_or_record }}): ChannelSubscription<{{ ch.params_type_or_record }}, {{ ch.django_msg_type_or_never }}, {{ ch.react_msg_type_or_never }}> {
+{% for ch in param_channels -%}
+export function use{{ ch.pascal_name }}Channel(params: {{ ch.params_type }}): ChannelSubscription<{{ ch.params_type }}, {{ ch.server_message_type }}, {{ ch.client_message_type }}> {
     return useChannel('{{ ch.name }}', params)
 }
-{% else -%}
-export function use{{ ch.pascal_name }}Channel(): ChannelSubscription<Record<string, never>, {{ ch.django_msg_type_or_never }}, {{ ch.react_msg_type_or_never }}> {
+
+{% endfor -%}
+{% for ch in paramless_channels -%}
+export function use{{ ch.pascal_name }}Channel(): ChannelSubscription<Record<string, never>, {{ ch.server_message_type }}, {{ ch.client_message_type }}> {
     return useChannel('{{ ch.name }}', {})
 }
-{% endif %}
+
 {% endfor -%}

protocol/mizan-codegen/templates/channels/channels.ts.j2

@@ -1,25 +1,15 @@
-// AUTO-GENERATED by mizan — do not edit
+{% for schema in schemas %}{{ schema }}{% if !loop.last %}
 
-{{ schemas_block }}
-// ── Channel Registry ──────────────────────────────────────────────────────
+{% endif %}{% endfor %}
 
 export const CHANNELS = {
 {%- for ch in channels %}
-    {{ ch.name }}: {
+    '{{ ch.name }}': {
         name: '{{ ch.name }}',
         pascalName: '{{ ch.pascal_name }}',
-        hasParams: {{ ch.has_params }},
-        hasReactMessage: {{ ch.has_react_message }},
-        hasDjangoMessage: {{ ch.has_django_message }},
-{%- if ch.has_params %}
-        paramsType: '{{ ch.params_type }}',
-{%- endif %}
-{%- if ch.has_react_message %}
-        reactMessageType: '{{ ch.react_message_type }}',
-{%- endif %}
-{%- if ch.has_django_message %}
-        djangoMessageType: '{{ ch.django_message_type }}',
-{%- endif %}
+{%- for slot in ch.slots %}
+        {{ slot.registry_key }}: '{{ slot.type_name }}',
+{%- endfor %}
     },
 {%- endfor %}
 } as const