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/test_codegen_parity.py

@@ -1,14 +1,9 @@
-"""
-AFI conformance — same fixture, same Mizan IR (KDL), all three adapters.
+"""Byte-equivalence of the Mizan IR (KDL) that mizan-django, mizan-fastapi,
+and the Rust mizan-axum backend emit for one shared fixture.
 
-Gates that mizan-django, mizan-fastapi, and the Rust mizan-axum backend
-emit byte-equivalent KDL for the same registered functions. The IR is the
-contract; whatever language wrote the backend, the wire/codegen-facing
-artifact is identical.
-
-Substrate-level gate, not e2e. Catches adapter symmetry problems —
-type-introspection divergence, ordering non-determinism — across all
-backends in one place.
+Each adapter is reached the way its own runtime reaches it: Django through
+its management command in a subprocess, FastAPI in-process against a cleared
+registry, Rust through the `export-ir` bin.
 """
 
 from __future__ import annotations
@@ -27,7 +22,6 @@ RUST_APP_DIR = HERE / "rust_app"
 
 
 def _fetch_django_ir() -> str:
-    """Spawn Django's management command and parse stdout as KDL."""
     result = subprocess.run(
         [sys.executable, str(DJANGO_MANAGE), "export_mizan_ir"],
         capture_output=True,
@@ -43,7 +37,7 @@ def _fetch_django_ir() -> str:
 
 
 def _fetch_fastapi_ir() -> str:
-    """Build the FastAPI app inline (fresh registry) and call build_ir()."""
+    # tests/afi/ must be importable for `fixture` and `fastapi_app` to resolve.
     sys.path.insert(0, str(HERE))
     try:
         from mizan_core.registry import clear_registry
@@ -58,7 +52,6 @@ def _fetch_fastapi_ir() -> str:
 
 
 def _fetch_rust_ir() -> str:
-    """Spawn the Rust `export-ir` bin and capture stdout."""
     result = subprocess.run(
         [
             "cargo", "run",
@@ -93,23 +86,20 @@ def django_ir() -> str:
 
 
 def test_fastapi_matches_rust(fastapi_ir: str, rust_ir: str) -> None:
-    """FastAPI ≡ Rust. The Mizan IR is the contract across languages."""
     assert fastapi_ir == rust_ir, (
         "FastAPI and Rust emit divergent Mizan IR for the same registered "
-        "functions. Substrate gate is red."
+        "functions and channels."
     )
 
 
 def test_django_matches_fastapi(django_ir: str, fastapi_ir: str) -> None:
-    """Django ≡ FastAPI."""
     assert django_ir == fastapi_ir, (
-        "Django and FastAPI emit divergent Mizan IR for the same "
-        "registered functions. Substrate gate is red."
+        "Django and FastAPI emit divergent Mizan IR for the same registered "
+        "functions and channels."
     )
 
 
 def test_all_three_match(django_ir: str, fastapi_ir: str, rust_ir: str) -> None:
-    """All three backends emit byte-identical KDL."""
     assert django_ir == fastapi_ir == rust_ir, (
         "Three-way IR divergence — see test_django_matches_fastapi and "
         "test_fastapi_matches_rust for which pair drifts."