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>

cores/mizan-rust-ssr/tests/fixture/Hello.js

@@ -1,7 +1,6 @@
 import { createElement } from "react"
 
-// A trivial component: props in, element out. The keystone only needs to prove
-// a real React tree renders to HTML inside a bare JS context.
+// Props in, element out. The `id` is the handle the render assertions match on.
 export function Hello({ name }) {
   return createElement("div", { id: "greeting" }, `Hello, ${name}!`)
 }

cores/mizan-rust-ssr/tests/fixture/entry.js

@@ -2,7 +2,7 @@ import { renderToStaticMarkup } from "react-dom/server.browser"
 import { createElement } from "react"
 import { Hello } from "./Hello.js"
 
-// The bundle exposes one global the embedded engine calls. No module system at
-// runtime — the engine receives a bare script that defines `renderApp`. This is
-// the production shape in miniature: build-time bundle, runtime eval.
+// There is no module system in the embedded engine — it receives a bare
+// script, so the entry point has to land on `globalThis` for the Rust side to
+// reach it.
 globalThis.renderApp = (props) => renderToStaticMarkup(createElement(Hello, props))

cores/mizan-rust-ssr/tests/fixture/runner.cjs

@@ -1,14 +1,11 @@
-// Proxy for the embedded-V8 runtime: a bare global context with no Node
-// builtins. Load the IIFE bundle (which assigns globalThis.renderApp) and call
-// it. What renders here renders in rusty_v8 — the engine swaps, the contract
-// (bundle defines a global render fn over a bare context) does not.
+// Runs bundle.js inside a `vm` context holding only the globals listed below,
+// so the bundle sees the same bare environment the embedded V8 engine gives it.
 const fs = require("fs")
 const vm = require("vm")
 
 const code = fs.readFileSync(__dirname + "/bundle.js", "utf8")
 
-// The minimal host globals React's bundle touches at init / sync render. The
-// rusty_v8 engine must provide the same set — this list is the spec for it.
+// The host globals React's bundle touches at init and during a sync render.
 const sandbox = {
   console, setTimeout, clearTimeout, queueMicrotask, MessageChannel, performance,
   TextEncoder, TextDecoder,
@@ -19,11 +16,12 @@ vm.createContext(sandbox)
 vm.runInContext(code, sandbox)
 
 const html = sandbox.renderApp({ name: "World" })
-console.log("RENDERED:", html)
-
 const expected = '<div id="greeting">Hello, World!</div>'
 if (html !== expected) {
-  console.error("MISMATCH — expected:", expected)
+  console.error(`expected ${expected}, got ${html}`)
   process.exit(1)
 }
-console.log("OK — React bundle renders in a bare JS context (V8 proxy)")
+console.log(html)
+// The sandbox's MessageChannel holds an open handle, so the event loop never
+// drains on its own; exit once the render has been checked.
+process.exit(0)