Mizan codegen substrate: Rust kernel + Rust codegen binary, JS generator deleted

The Mizan codegen substrate moves off JavaScript template-literal emission
onto a compiled Rust binary that consumes the same OpenAPI + x-mizan-* IR
the JS substrate consumed. Three structural wins fall out of one move:

1. Moat closes. The codegen logic (how `affects` becomes auto-invalidation,
   how named contexts collapse onto bundled fetches, how the registry-to-
   Provider mapping is shaped) ships compiled instead of as source bytes
   in every consumer's node_modules.

2. Pattern F (lines.push append-walls) becomes structurally unauthorable.
   The emit substrate is askama templates in templates/<target>/*.j2 —
   actual target-language files with {{ ... }} substitution markers,
   syntax-highlighted natively, type-checked against the render context
   structs at compile time. The Rust emit modules build typed render
   contexts and call .render(); no string-builder surface exists.

3. OpenAPI `default`-bearing fields now emit as non-optional in TS / Python
   / Rust — the server always populates them, so consumer code reads them
   without nullable checks. Surfaced by Blazr's typecheck on regeneration.

Layout:
  frontends/mizan-rust/        — Rust port of @mizan/base; #[cfg(feature="pyo3")]
                                 exposes PyMizanClient for the Python target.
  protocol/mizan-codegen/      — codegen binary source + askama templates.
  protocol/mizan-generate/     — npm-package shim. bin/launcher.mjs dispatches
                                 to the platform-appropriate prebuilt binary.
                                 Old generator/ JS tree deleted.
  tests/rust/                  — wire-parity drivers. drive_kernel exercises
                                 raw client.call() / fetch_context(); drive_emitted
                                 exercises the typed crate the codegen emits.
  tests/afi/afi_codegen_app.py — codegen entrypoint module (imports + registers).
  backends/mizan-fastapi/.../schema.py — adds outputNullable so the Rust
                                 codegen can wrap T | None responses in Option<T>.

Verification:
  - 20 mizan-codegen tests green (IR deserialization, byte-equivalent
    parity vs JS baseline for stage1/rust/python/react/vue/svelte,
    structural test for channels).
  - tests/rust/run_wire_parity.py — 12/12 probes green via the Rust binary
    driving the FastAPI fixture end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,3 @@
+// AUTO-GENERATED by mizan — do not edit
+
+pub mod user;

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

@@ -0,0 +1,29 @@
+// 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};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UserContextData {
+    pub user_profile: UserProfileOutput,
+    pub user_orders: UserOrdersOutput,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UserContextParams {
+    pub user_id: i64,
+}
+
+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 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

@@ -0,0 +1,14 @@
+// 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 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

@@ -0,0 +1,14 @@
+// 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 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

@@ -0,0 +1,6 @@
+// AUTO-GENERATED by mizan — do not edit
+
+pub mod echo;
+pub mod find_user;
+pub mod rename_user;
+pub mod whoami;

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

@@ -0,0 +1,14 @@
+// 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 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

@@ -0,0 +1,14 @@
+// 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 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

@@ -0,0 +1,8 @@
+// AUTO-GENERATED by mizan — do not edit
+
+pub mod types;
+pub mod contexts;
+pub mod mutations;
+pub mod functions;
+
+pub use mizan_rust::{MizanClient, MizanConfig, MizanError};

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

@@ -0,0 +1,3 @@
+// AUTO-GENERATED by mizan — do not edit
+
+pub mod update_profile;

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

@@ -0,0 +1,14 @@
+// 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 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

@@ -0,0 +1,98 @@
+// AUTO-GENERATED by mizan — do not edit
+
+#![allow(non_camel_case_types)]
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct HTTPValidationError {
+    pub detail: Option<Vec<ValidationError>>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct OrderOutput {
+    pub id: i64,
+    pub user_id: i64,
+    pub total: i64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ValidationError {
+    pub loc: Vec<serde_json::Value>,
+    pub msg: String,
+    #[serde(rename = "type")]
+    pub r#type: String,
+    pub input: Option<serde_json::Value>,
+    pub ctx: Option<serde_json::Value>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct EchoInput {
+    pub text: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct EchoOutput {
+    pub message: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct FindUserInput {
+    pub user_id: i64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct FindUserOutput {
+    pub user_id: i64,
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct RenameUserInput {
+    pub user_id: i64,
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct RenameUserOutput {
+    pub user_id: i64,
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UpdateProfileInput {
+    pub user_id: i64,
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UpdateProfileOutput {
+    pub ok: bool,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UserOrdersInput {
+    pub user_id: i64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct UserOrdersOutput(pub Vec<OrderOutput>);
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UserProfileInput {
+    pub user_id: i64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UserProfileOutput {
+    pub user_id: i64,
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct WhoamiOutput {
+    pub email: String,
+    pub authenticated: bool,
+}
+