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/templates/rust/Cargo.toml.j2

@@ -4,7 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
-mizan-rust = {{ kernel_dep }}
+mizan-rust = { {% for kv in kernel_dep %}{% if !loop.first %}, {% endif %}{{ kv.key }} = "{{ kv.value }}"{% endfor %} }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["rt", "macros"] }

protocol/mizan-codegen/templates/rust/call.rs.j2

@@ -1,15 +1,12 @@
-// AUTO-GENERATED by mizan — do not edit
-
-use serde_json::Value;
-
 use mizan_rust::{MizanClient, MizanError};
 
 {% if !type_imports.is_empty() -%}
 use crate::types::{ {{- type_imports|join(", ") -}} };
 
 {% endif -%}
-pub async fn call_{{ snake }}(client: &MizanClient{{ input_param }}) -> Result<{{ return_type }}, MizanError> {
-    let args_value = {{ args_value }};
+pub async fn call_{{ snake }}(client: &MizanClient{% match input %}{% when CallInput::Typed with (t) %}, args: &{{ crate::emit::casing::rust_type_ident(t) }}{% when CallInput::Absent %}{% endmatch %}) -> Result<{% if nullable %}Option<{{ output_type }}>{% else %}{{ output_type }}{% endif %}, MizanError> {
+    let args_value = {% match input %}{% when CallInput::Typed with (t) %}serde_json::to_value(args)
+        .map_err(|e| MizanError::transport(format!("encode {{ name }} args: {e}")))?{% when CallInput::Absent %}serde_json::Value::Object(Default::default()){% endmatch %};
     let raw = client.call("{{ name }}", args_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode {{ name }} result: {e}")))

protocol/mizan-codegen/templates/rust/context.rs.j2

@@ -1,7 +1,4 @@
-// AUTO-GENERATED by mizan — do not edit
-
 use serde::{Deserialize, Serialize};
-use serde_json::Value;
 
 use mizan_rust::{MizanClient, MizanError};
 
@@ -12,16 +9,16 @@ use crate::types::{ {{- type_imports|join(", ") -}} };
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct {{ pascal }}ContextData {
 {% for field in data_fields -%}
-{% if field.has_rename %}    #[serde(rename = "{{ field.raw_name }}")]
-{% endif %}    pub {{ field.ident }}: {{ field.ty }},
+{% if field.has_rename %}    #[serde(rename = "{{ field.wire_name }}")]
+{% endif %}    pub {{ field.ident }}: {% if field.optional %}Option<{{ field.ty }}>{% else %}{{ field.ty }}{% endif %},
 {% endfor -%}
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct {{ pascal }}ContextParams {
 {% for p in params -%}
-{% if p.has_rename %}    #[serde(rename = "{{ p.raw_name }}")]
-{% endif %}    pub {{ p.ident }}: {{ p.ty }},
+{% if p.has_rename %}    #[serde(rename = "{{ p.wire_name }}")]
+{% endif %}    pub {{ p.ident }}: {% if p.optional %}Option<{{ p.ty }}>{% else %}{{ p.ty }}{% endif %},
 {% endfor -%}
 }
 
@@ -29,7 +26,8 @@ pub async fn fetch_{{ snake }}_context(
     client: &MizanClient,
     params: &{{ pascal }}ContextParams,
 ) -> Result<{{ pascal }}ContextData, 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 {{ ctx_name }} context params: {e}")))?;
     let raw = client.fetch_context("{{ ctx_name }}", &params_value).await?;
     serde_json::from_value(raw)
         .map_err(|e| MizanError::transport(format!("decode {{ ctx_name }} context: {e}")))

protocol/mizan-codegen/templates/rust/lib.rs.j2

@@ -1,5 +1,3 @@
-// AUTO-GENERATED by mizan — do not edit
-
 pub mod types;
 {% if has_contexts %}pub mod contexts;
 {% endif %}{% if has_mutations %}pub mod mutations;

protocol/mizan-codegen/templates/rust/mod.rs.j2

@@ -1,5 +1,3 @@
-// AUTO-GENERATED by mizan — do not edit
-
 {% for name in modules -%}
 pub mod {{ name }};
 {% endfor %}

protocol/mizan-codegen/templates/rust/schema.rs.j2

@@ -0,0 +1,20 @@
+{%- match kind -%}
+{%- when RustSchemaKind::Struct with (fields) -%}
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct {{ name }} {
+{% for f in fields %}{% if f.has_rename %}    #[serde(rename = "{{ f.wire_name }}")]
+{% endif %}    pub {{ f.ident }}: {{ f.ty }},
+{% endfor %}}
+{%- when RustSchemaKind::StringEnum with (variants) -%}
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub enum {{ name }} {
+{% for v in variants %}{% if v.has_rename %}    #[serde(rename = "{{ v.wire_name }}")]
+{% endif %}    {{ v.ident }},
+{% endfor %}}
+{%- when RustSchemaKind::TransparentArray with (inner) -%}
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct {{ name }}(pub Vec<{{ inner }}>);
+{%- when RustSchemaKind::Alias with (inner) -%}
+pub type {{ name }} = {{ inner }};
+{%- endmatch -%}

protocol/mizan-codegen/templates/rust/type_expr.rs.j2

@@ -0,0 +1,10 @@
+{%- match shape -%}
+{%- when RustShape::Named with (name) -%}
+{{ name }}
+{%- when RustShape::List with (inner) -%}
+Vec<{{ crate::emit::rust::rust_type(inner) }}>
+{%- when RustShape::Optional with (inner) -%}
+Option<{{ crate::emit::rust::rust_type(inner) }}>
+{%- when RustShape::Json -%}
+serde_json::Value
+{%- endmatch -%}

protocol/mizan-codegen/templates/rust/types.rs.j2

@@ -1,8 +1,9 @@
-// AUTO-GENERATED by mizan — do not edit
-
 #![allow(non_camel_case_types)]
 
 use serde::{Deserialize, Serialize};
 
-{{ schemas_block }}
-{{ hoisted_enums_block }}
+{% for schema in schemas %}{{ schema }}
+
+{% endfor %}{% for hoisted in hoisted_enums %}{{ hoisted }}
+{% if !loop.last %}
+{% endif %}{% endfor %}