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>

examples/fastapi-react-site/mizan.spec.ts

@@ -1,9 +1,7 @@
 /**
- * mizan-fastapi e2e — Real Chromium → React with generated hooks → real FastAPI server.
- *
- * Mirrors examples/django-react-site/mizan.spec.ts minus channel/form tests
- * (those features are Django-only — FastAPI projects use native equivalents
- * or skip them entirely).
+ * Drives a real Chromium against the harness (React + generated hooks) talking
+ * to a real FastAPI server. Each test navigates to a URL hash that selects one
+ * fixture component, then reads that component's data-testid nodes.
  */
 
 import { test, expect } from '@playwright/test'
@@ -104,14 +102,14 @@ test.describe('error codes from generated hooks', () => {
         expect(['UNAUTHORIZED', 'FORBIDDEN']).toContain(error!.code)
     })
 
-    test('useNotImplementedFn → NOT_IMPLEMENTED', async ({ page }) => {
+    test('useEchoTransform with an unregistered mode → NOT_IMPLEMENTED', async ({ page }) => {
         await fixture(page, 'not-implemented')
         const error = await getError(page)
         expect(error!.type).toBe('MizanError')
         expect(error!.code).toBe('NOT_IMPLEMENTED')
     })
 
-    test('useBuggyFn → INTERNAL_ERROR', async ({ page }) => {
+    test('useDivide by zero → INTERNAL_ERROR', async ({ page }) => {
         await fixture(page, 'internal-error')
         const error = await getError(page)
         expect(error!.type).toBe('MizanError')
@@ -160,10 +158,8 @@ test.describe('merge protocol', () => {
         // The morphs context bundles two list slots:
         //   morph_groups: list[MorphGroupMeta]  — {id, label, count}
         //   morph_layers: list[MorphLayer]      — {id, group_id, label, value}
-        // set_morph_value returns MorphLayer. Server-side slot resolution
-        // (via mizan_core.type_utils.types_match_for_merge) must route to
-        // morph_layers and leave morph_groups intact. A kernel-side heuristic
-        // would have to guess between the two id-bearing list slots.
+        // set_morph_value returns MorphLayer, so the server resolves the slot
+        // by return type and the value lands in morph_layers.
         const morphsFetches: string[] = []
         page.on('request', (req) => {
             const url = req.url()
@@ -172,21 +168,20 @@ test.describe('merge protocol', () => {
 
         await page.goto(`${BASE}#merge-morph`)
         await page.waitForFunction(() => {
+            // The fixture renders a plain "loading..." div until both slots
+            // arrive, so this node exists only once its content is full JSON.
             const el = document.querySelector('[data-testid="result"]')
             if (!el) return false
-            try {
-                const data = JSON.parse(el.textContent!)
-                return data.layers?.some((l: any) => l.id === 1 && l.value === 0.75)
-            } catch { return false }
+            const data = JSON.parse(el.textContent!)
+            return data.layers?.some((l: any) => l.id === 1 && l.value === 0.75)
         }, { timeout: 5000 })
 
         const result = await getResult(page)
         const layer = result.layers.find((l: any) => l.id === 1)
         expect(layer.value).toBe(0.75)
         expect(layer.label).toBe('brow')
-        // Sibling slot is unchanged — the server didn't route MorphLayer into morph_groups.
         expect(result.groups).toEqual([{ id: 1, label: 'face', count: 2 }])
-        // Initial mount fetches once; merge path must not trigger a refetch.
+        // Initial mount fetches once; the merge path must not trigger a refetch.
         expect(morphsFetches.length).toBe(1)
     })
 })