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/src/drive_emitted.rs

@@ -0,0 +1,75 @@
+//! Drive the codegen-emitted `fixture_client` crate against a live
+//! FastAPI fixture. Validates not just the kernel wire (which
+//! `drive_kernel.rs` already covers) but that the codegen actually
+//! produces typed-functions that round-trip cleanly through the same
+//! kernel.
+
+use std::env;
+use std::process::ExitCode;
+
+use mizan_rust::{MizanClient, MizanConfig};
+
+use fixture_client::contexts::user::{fetch_user_context, UserContextParams};
+use fixture_client::functions::echo::call_echo;
+use fixture_client::functions::find_user::call_find_user;
+use fixture_client::functions::rename_user::call_rename_user;
+use fixture_client::functions::whoami::call_whoami;
+use fixture_client::mutations::update_profile::call_update_profile;
+use fixture_client::types::{
+    EchoInput, FindUserInput, RenameUserInput, UpdateProfileInput,
+};
+
+
+#[tokio::main]
+async fn main() -> ExitCode {
+    let base_url = env::args().nth(1)
+        .unwrap_or_else(|| "http://127.0.0.1:8765/api/mizan".to_string());
+    let client = MizanClient::new(MizanConfig {
+        base_url,
+        session: false,
+        ..Default::default()
+    });
+
+    let mut failures = 0usize;
+
+    match call_echo(&client, &EchoInput { text: "hello".to_string() }).await {
+        Ok(out) => println!("call_echo -> message={:?}", out.message),
+        Err(e) => { eprintln!("call_echo ERR {e}"); failures += 1; }
+    }
+
+    match call_whoami(&client).await {
+        Ok(out) => println!("call_whoami -> authenticated={} email={:?}", out.authenticated, out.email),
+        Err(e) => { eprintln!("call_whoami ERR {e}"); failures += 1; }
+    }
+
+    match call_find_user(&client, &FindUserInput { user_id: 99999 }).await {
+        Ok(None) => println!("call_find_user(99999) -> None"),
+        Ok(Some(out)) => println!("call_find_user(99999) -> user_id={} name={:?}", out.user_id, out.name),
+        Err(e) => { eprintln!("call_find_user ERR {e}"); failures += 1; }
+    }
+
+    match call_update_profile(&client, &UpdateProfileInput { user_id: 5, name: "Ryth".to_string() }).await {
+        Ok(out) => println!("call_update_profile -> ok={}", out.ok),
+        Err(e) => { eprintln!("call_update_profile ERR {e}"); failures += 1; }
+    }
+
+    match call_rename_user(&client, &RenameUserInput { user_id: 5, name: "RythR".to_string() }).await {
+        Ok(out) => println!("call_rename_user -> user_id={} name={:?}", out.user_id, out.name),
+        Err(e) => { eprintln!("call_rename_user ERR {e}"); failures += 1; }
+    }
+
+    match fetch_user_context(&client, &UserContextParams { user_id: 5 }).await {
+        Ok(out) => println!(
+            "fetch_user_context(5) -> user_profile={{ user_id:{}, name:{:?} }} user_orders.len={}",
+            out.user_profile.user_id, out.user_profile.name, out.user_orders.0.len(),
+        ),
+        Err(e) => { eprintln!("fetch_user_context ERR {e}"); failures += 1; }
+    }
+
+    if failures > 0 {
+        eprintln!("[drive_emitted] {failures} probe(s) failed");
+        ExitCode::FAILURE
+    } else {
+        ExitCode::SUCCESS
+    }
+}

tests/rust/src/drive_kernel.rs

@@ -0,0 +1,74 @@
+//! Drive the mizan-rust kernel against a live FastAPI fixture app and
+//! print every response. Used by `run_wire_parity.sh` which:
+//!
+//! 1. Boots `tests/afi/fastapi_app.py` via uvicorn on port 8765.
+//! 2. Polls `/openapi.json` until the server is up.
+//! 3. Runs `cargo run --bin drive_kernel -- http://127.0.0.1:8765/api/mizan`.
+//! 4. Diffs the stdout against a committed snapshot.
+//!
+//! The kernel exercises every endpoint the fixture declares: the two
+//! plain functions (`echo`, `whoami`), the two-function `user` context,
+//! the `update_profile` mutation, the `find_user` Optional path, and
+//! the `rename_user` merge mutation.
+
+use std::env;
+use std::process::ExitCode;
+
+use mizan_rust::{MizanClient, MizanConfig};
+use serde_json::{json, Value};
+
+
+#[tokio::main]
+async fn main() -> ExitCode {
+    let base_url = env::args().nth(1)
+        .unwrap_or_else(|| "http://127.0.0.1:8765/api/mizan".to_string());
+    let client = MizanClient::new(MizanConfig {
+        base_url,
+        session: false,
+        ..Default::default()
+    });
+
+    let mut failures = 0usize;
+    failures += probe(&client, "echo", json!({"text": "hello"})).await;
+    failures += probe(&client, "whoami", json!({})).await;
+    failures += probe(&client, "find_user", json!({"user_id": 99999})).await;
+    failures += probe(&client, "update_profile", json!({"user_id": 5, "name": "Ryth"})).await;
+    failures += probe(&client, "rename_user", json!({"user_id": 5, "name": "RythR"})).await;
+
+    failures += probe_context(&client, "user", json!({"user_id": 5})).await;
+
+    if failures > 0 {
+        eprintln!("[drive_kernel] {failures} probe(s) failed");
+        ExitCode::FAILURE
+    } else {
+        ExitCode::SUCCESS
+    }
+}
+
+
+async fn probe(client: &MizanClient, fn_name: &str, args: Value) -> usize {
+    match client.call(fn_name, args.clone()).await {
+        Ok(result) => {
+            println!("call {fn_name} args={args} -> {result}");
+            0
+        }
+        Err(err) => {
+            eprintln!("call {fn_name} args={args} -> ERR {err}");
+            1
+        }
+    }
+}
+
+
+async fn probe_context(client: &MizanClient, name: &str, params: Value) -> usize {
+    match client.fetch_context(name, &params).await {
+        Ok(data) => {
+            println!("ctx {name} params={params} -> {data}");
+            0
+        }
+        Err(err) => {
+            eprintln!("ctx {name} params={params} -> ERR {err}");
+            1
+        }
+    }
+}