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-generate/bin/launcher.mjs

@@ -1,37 +1,47 @@
 #!/usr/bin/env node
-// Mizan codegen npm-package shim — dispatches to the platform-appropriate
-// `mizan-generate` Rust binary in this directory. Source for the binary
-// lives at `protocol/mizan-codegen/`; published releases ship one binary
-// per supported platform.
 
-import { spawn } from 'child_process'
+import { spawn, spawnSync } from 'child_process'
 import { fileURLToPath } from 'url'
 import { dirname, join } from 'path'
 import { existsSync } from 'fs'
 import { platform, arch } from 'os'
 
 const here = dirname(fileURLToPath(import.meta.url))
+const exeSuffix = platform() === 'win32' ? '.exe' : ''
+const codegenDir = join(here, '..', '..', 'mizan-codegen')
+const manifest = join(codegenDir, 'Cargo.toml')
 
-const platforms = {
-    'linux-x64':   'mizan-generate-linux-x64',
-    'darwin-arm64': 'mizan-generate-darwin-arm64',
-    'darwin-x64':  'mizan-generate-darwin-x64',
-    'win32-x64':   'mizan-generate-win32-x64.exe',
+function resolveBinary() {
+    if (existsSync(manifest)) {
+        // Rebuilding unconditionally is what keeps the binary in step with the
+        // crate; an up-to-date cargo build is a no-op, a stale target/release
+        // artifact is otherwise indistinguishable from a fresh one.
+        const build = spawnSync(
+            'cargo',
+            ['build', '--release', '--quiet', '--manifest-path', manifest],
+            { stdio: 'inherit' },
+        )
+        if (build.error) {
+            console.error(`[mizan-generate] cannot run cargo: ${build.error.message}`)
+            console.error(`[mizan-generate] cargo builds ${manifest}; install a Rust toolchain`)
+            process.exit(1)
+        }
+        if (build.status !== 0) {
+            console.error(`[mizan-generate] cargo build --release failed for ${manifest}`)
+            process.exit(build.status)
+        }
+        return join(codegenDir, 'target', 'release', `mizan-generate${exeSuffix}`)
+    }
+
+    const packaged = join(here, `mizan-generate-${platform()}-${arch()}${exeSuffix}`)
+    if (!existsSync(packaged)) {
+        console.error(`[mizan-generate] no binary packaged for ${platform()}-${arch()}: ${packaged}`)
+        process.exit(1)
+    }
+    return packaged
 }
 
-const key = `${platform()}-${arch()}`
-const binName = platforms[key]
-if (!binName) {
-    console.error(`[mizan-generate] no prebuilt binary for ${key}`)
-    process.exit(1)
-}
-
-const binPath = join(here, binName)
-if (!existsSync(binPath)) {
-    console.error(`[mizan-generate] binary missing: ${binPath}`)
-    console.error('[mizan-generate] build from source: cargo build --release --manifest-path <mizan>/protocol/mizan-codegen/Cargo.toml')
-    process.exit(1)
-}
+const binPath = resolveBinary()
 
 const child = spawn(binPath, process.argv.slice(2), { stdio: 'inherit' })
 child.on('exit', code => process.exit(code ?? 1))