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-macros/src/lib.rs

@@ -1,47 +1,34 @@
-//! Proc macros for `mizan-core`. See sibling modules for each macro's body.
+//! Proc macros for `mizan-core`. See sibling modules for each macro's body:
+//! `derive` for `#[derive(Mizan)]`, `context` / `function` / `channel` for the
+//! three attribute macros, `shape` for the shared `syn::Type` lowering.
 //!
-//! Consumer code reads:
-//!   ```ignore
-//!   use mizan_core::prelude::*;
-//!   pub use mizan_core as mizan;  // so `#[mizan::context]` / `#[mizan::client]` read naturally
-//!
-//!   #[derive(Mizan, serde::Serialize, serde::Deserialize)]
-//!   pub struct ProfileOutput { pub user_id: i64, pub name: String }
-//!
-//!   #[mizan::context("user")]
-//!   pub struct UserCtx;
-//!
-//!   #[mizan::client(context = UserCtx)]
-//!   pub async fn user_profile(req: &Request, user_id: i64) -> ProfileOutput { ... }
-//!   ```
-//!
-//! The function macro is named `client` to mirror Python's `@client`
-//! decorator and to keep the namespace `mizan::` purely a module path —
-//! `#[mizan(...)]` would collide with `mizan::context` (a module path
-//! can't simultaneously be a callable macro in Rust).
+//! The function macro is named `client` so `mizan::` stays purely a module
+//! path — a module path can't simultaneously be a callable macro in Rust, so
+//! `#[mizan(...)]` would collide with `mizan::context`.
 
+mod channel;
 mod context;
 mod derive;
 mod function;
 mod shape;
 
 use proc_macro::TokenStream;
-use syn::{parse_macro_input, DeriveInput, ItemFn, ItemStruct};
+use syn::{parse_macro_input, ItemStruct};
 
 #[proc_macro_derive(Mizan)]
 pub fn derive_mizan(input: TokenStream) -> TokenStream {
-    let input = parse_macro_input!(input as DeriveInput);
-    derive::expand(input).into()
+    let derived = parse_macro_input!(input as derive::MizanDerive);
+    derive::expand(derived).into()
 }
 
 #[proc_macro_attribute]
 pub fn context(attr: TokenStream, item: TokenStream) -> TokenStream {
-    let args = match context::ContextArgs::parse(attr.into()) {
-        Ok(a) => a,
+    let name = match context::ContextName::parse(attr.into()) {
+        Ok(n) => n,
         Err(e) => return e.to_compile_error().into(),
     };
     let item = parse_macro_input!(item as ItemStruct);
-    context::expand(args, item).into()
+    context::expand(name, item).into()
 }
 
 /// The function-registration attribute macro. Used as `#[mizan::client]`
@@ -49,10 +36,17 @@ pub fn context(attr: TokenStream, item: TokenStream) -> TokenStream {
 /// websocket, private)]`.
 #[proc_macro_attribute]
 pub fn client(attr: TokenStream, item: TokenStream) -> TokenStream {
-    let args = match function::FunctionArgs::parse(attr.into()) {
-        Ok(a) => a,
-        Err(e) => return e.to_compile_error().into(),
-    };
-    let item = parse_macro_input!(item as ItemFn);
-    function::expand(args, item).into()
+    let args = parse_macro_input!(attr as function::FunctionArgs);
+    let handler = parse_macro_input!(item as function::Handler);
+    function::expand(args, handler).into()
+}
+
+/// The channel-registration attribute macro. Used as
+/// `#[mizan::channel("<wire-name>", params = P, client_message = C,
+/// server_message = S)]` on a unit struct; every slot is optional.
+#[proc_macro_attribute]
+pub fn channel(attr: TokenStream, item: TokenStream) -> TokenStream {
+    let args = parse_macro_input!(attr as channel::ChannelArgs);
+    let item = parse_macro_input!(item as ItemStruct);
+    channel::expand(args, item).into()
 }