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/rust/run_wire_parity.py

@@ -1,20 +1,13 @@
-"""Three-way wire-parity check.
+"""Run the wire-parity drivers against each backend.
 
 For each backend (FastAPI, Rust axum):
   1. Boot the fixture server on a free port.
-  2. Poll /api/mizan/session/ until the server responds.
+  2. Poll the readiness surface until the server responds.
   3. Run the Rust `drive_kernel` binary (raw kernel calls) against it.
   4. Run the Rust `drive_emitted` binary (typed codegen functions) against it.
   5. Tear the server down.
 
-Any non-zero driver exit propagates as the script's exit code. Adding the
-Rust backend here proves that mizan-axum honors the same wire contract as
-mizan-fastapi — same JSON shapes, same invalidate/merge semantics — beyond
-the static IR equivalence the codegen-parity test gates.
-
-Readiness probe is `/api/mizan/session/` (Mizan-protocol-shaped) rather
-than `/openapi.json` (FastAPI-feature-shaped) so the harness reads the
-same surface across backends.
+Any non-zero driver exit propagates as the script's exit code.
 """
 
 from __future__ import annotations
@@ -45,6 +38,7 @@ def pick_free_port() -> int:
 
 def wait_for_server(port: int, timeout_s: float, label: str) -> bool:
     deadline = time.monotonic() + timeout_s
+    # every backend serves /session/; /openapi.json exists only on FastAPI
     url = f"http://127.0.0.1:{port}/api/mizan/session/"
     while time.monotonic() < deadline:
         try: