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/tests/fixtures/toolchain/drivers/react.tsx

@@ -0,0 +1,59 @@
+import {
+    MizanContext,
+    UserContext,
+    useEcho,
+    useFindUser,
+    useMizan,
+    useRenameUser,
+    useUpdateProfile,
+    useUserContext,
+    useUserOrders,
+    useUserProfile,
+    useWhoami,
+    type ContextState,
+} from '../src/react'
+import type { UserContextData } from '../src/index'
+
+export function Screen() {
+    return (
+        <MizanContext baseUrl="/api/mizan" session={false}>
+            <UserContext user_id={1}>
+                <Panel />
+            </UserContext>
+        </MizanContext>
+    )
+}
+
+function Panel() {
+    const context: ContextState<UserContextData> = useUserContext()
+    const orders = useUserOrders()
+    const profile = useUserProfile()
+
+    const echo = useEcho()
+    const findUser = useFindUser()
+    const renameUser = useRenameUser()
+    const updateProfile = useUpdateProfile()
+    const whoami = useWhoami()
+    const { call, fetch } = useMizan()
+
+    async function run() {
+        const echoed = await echo.mutate({ text: 'hello' })
+        const found = await findUser.mutate({ user_id: 1 })
+        const renamed = await renameUser.mutate({ user_id: 1, name: 'renamed' })
+        const updated = await updateProfile.mutate({ user_id: 1, name: 'renamed' })
+        const identity = await whoami.mutate()
+        const raw = await call('echo', { text: 'hello' })
+        const bundle = await fetch('user', { user_id: 1 })
+        console.log(echoed.message, found.name, renamed.name, updated.ok,
+                    identity.authenticated, raw, bundle)
+    }
+
+    const pending: boolean = echo.isPending
+    const failure: Error | null = echo.error
+
+    return (
+        <button onClick={run} disabled={pending}>
+            {context.status} {orders?.length ?? 0} {profile?.name ?? ''} {failure?.message ?? ''}
+        </button>
+    )
+}