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/fixture_client/.gitattributes

@@ -0,0 +1 @@
+* linguist-generated=true

tests/rust/fixture_client/src/contexts/mod.rs

@@ -1,3 +1 @@
-// AUTO-GENERATED by mizan — do not edit
-
 pub mod user;

tests/rust/fixture_client/src/contexts/user.rs

@@ -1,16 +1,13 @@
-// AUTO-GENERATED by mizan — do not edit
-
 use serde::{Deserialize, Serialize};
-use serde_json::Value;
 
 use mizan_rust::{MizanClient, MizanError};
 
-use crate::types::{UserProfileOutput, UserOrdersOutput};
+use crate::types::{UserOrdersOutput, UserProfileOutput};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct UserContextData {
-    pub user_profile: UserProfileOutput,
     pub user_orders: UserOrdersOutput,
+    pub user_profile: UserProfileOutput,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -22,7 +19,8 @@ pub async fn fetch_user_context(
     client: &MizanClient,
     params: &UserContextParams,
 ) -> Result<UserContextData, MizanError> {
-    let params_value = serde_json::to_value(params).unwrap_or(Value::Object(Default::default()));
+    let params_value = serde_json::to_value(params)
+        .map_err(|e| MizanError::transport(format!("encode user context params: {e}")))?;
     let raw = client.fetch_context("user", &params_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode user context: {e}")))

tests/rust/fixture_client/src/functions/echo.rs

@@ -1,13 +1,10 @@
-// AUTO-GENERATED by mizan — do not edit
-
-use serde_json::Value;
-
 use mizan_rust::{MizanClient, MizanError};
 
 use crate::types::{EchoOutput, EchoInput};
 
 pub async fn call_echo(client: &MizanClient, args: &EchoInput) -> Result<EchoOutput, MizanError> {
-    let args_value = serde_json::to_value(args).unwrap_or(Value::Object(Default::default()));
+    let args_value = serde_json::to_value(args)
+        .map_err(|e| MizanError::transport(format!("encode echo args: {e}")))?;
     let raw = client.call("echo", args_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode echo result: {e}")))

tests/rust/fixture_client/src/functions/find_user.rs

@@ -1,13 +1,10 @@
-// AUTO-GENERATED by mizan — do not edit
-
-use serde_json::Value;
-
 use mizan_rust::{MizanClient, MizanError};
 
 use crate::types::{FindUserOutput, FindUserInput};
 
 pub async fn call_find_user(client: &MizanClient, args: &FindUserInput) -> Result<Option<FindUserOutput>, MizanError> {
-    let args_value = serde_json::to_value(args).unwrap_or(Value::Object(Default::default()));
+    let args_value = serde_json::to_value(args)
+        .map_err(|e| MizanError::transport(format!("encode find_user args: {e}")))?;
     let raw = client.call("find_user", args_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode find_user result: {e}")))

tests/rust/fixture_client/src/functions/mod.rs

@@ -1,5 +1,3 @@
-// AUTO-GENERATED by mizan — do not edit
-
 pub mod echo;
 pub mod find_user;
 pub mod rename_user;

tests/rust/fixture_client/src/functions/rename_user.rs

@@ -1,13 +1,10 @@
-// AUTO-GENERATED by mizan — do not edit
-
-use serde_json::Value;
-
 use mizan_rust::{MizanClient, MizanError};
 
 use crate::types::{RenameUserOutput, RenameUserInput};
 
 pub async fn call_rename_user(client: &MizanClient, args: &RenameUserInput) -> Result<RenameUserOutput, MizanError> {
-    let args_value = serde_json::to_value(args).unwrap_or(Value::Object(Default::default()));
+    let args_value = serde_json::to_value(args)
+        .map_err(|e| MizanError::transport(format!("encode rename_user args: {e}")))?;
     let raw = client.call("rename_user", args_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode rename_user result: {e}")))

tests/rust/fixture_client/src/functions/whoami.rs

@@ -1,13 +1,9 @@
-// AUTO-GENERATED by mizan — do not edit
-
-use serde_json::Value;
-
 use mizan_rust::{MizanClient, MizanError};
 
 use crate::types::{WhoamiOutput};
 
 pub async fn call_whoami(client: &MizanClient) -> Result<WhoamiOutput, MizanError> {
-    let args_value = Value::Object(Default::default());
+    let args_value = serde_json::Value::Object(Default::default());
     let raw = client.call("whoami", args_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode whoami result: {e}")))

tests/rust/fixture_client/src/lib.rs

@@ -1,5 +1,3 @@
-// AUTO-GENERATED by mizan — do not edit
-
 pub mod types;
 pub mod contexts;
 pub mod mutations;

tests/rust/fixture_client/src/mutations/mod.rs

@@ -1,3 +1 @@
-// AUTO-GENERATED by mizan — do not edit
-
 pub mod update_profile;

tests/rust/fixture_client/src/mutations/update_profile.rs

@@ -1,13 +1,10 @@
-// AUTO-GENERATED by mizan — do not edit
-
-use serde_json::Value;
-
 use mizan_rust::{MizanClient, MizanError};
 
 use crate::types::{UpdateProfileOutput, UpdateProfileInput};
 
 pub async fn call_update_profile(client: &MizanClient, args: &UpdateProfileInput) -> Result<UpdateProfileOutput, MizanError> {
-    let args_value = serde_json::to_value(args).unwrap_or(Value::Object(Default::default()));
+    let args_value = serde_json::to_value(args)
+        .map_err(|e| MizanError::transport(format!("encode update_profile args: {e}")))?;
     let raw = client.call("update_profile", args_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode update_profile result: {e}")))

tests/rust/fixture_client/src/types.rs

@@ -1,5 +1,3 @@
-// AUTO-GENERATED by mizan — do not edit
-
 #![allow(non_camel_case_types)]
 
 use serde::{Deserialize, Serialize};