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>

protocol/mizan-codegen/src/config.rs

@@ -0,0 +1,119 @@
+//! Codegen configuration — deserialized from `mizan.toml` at the consumer
+//! project root. Replaces the JS substrate's `mizan.config.mjs`.
+//!
+//! Example:
+//!
+//! ```toml
+//! project_id = "blazr-studio"
+//! output = "src/api"
+//! targets = ["react"]
+//!
+//! [source.fastapi]
+//! module = "blazr_session.handlers"
+//! cwd = "../.."
+//! command = ["uv", "run", "python"]
+//!
+//! [rust_kernel]
+//! path = "../../mizan/frontends/mizan-rust"
+//! ```
+
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+
+use serde::Deserialize;
+
+
+#[derive(Debug, Deserialize)]
+pub struct Config {
+    #[serde(default)]
+    pub project_id: Option<String>,
+
+    #[serde(default = "default_output")]
+    pub output: PathBuf,
+
+    #[serde(default = "default_targets")]
+    pub targets: Vec<String>,
+
+    #[serde(default)]
+    pub source: SourceConfig,
+
+    #[serde(default)]
+    pub rust_kernel: Option<RustKernelSpec>,
+
+    #[serde(default)]
+    pub rust_crate_name: Option<String>,
+}
+
+
+fn default_output() -> PathBuf {
+    PathBuf::from("src/api")
+}
+
+
+fn default_targets() -> Vec<String> {
+    vec!["react".to_string()]
+}
+
+
+#[derive(Debug, Deserialize, Default)]
+pub struct SourceConfig {
+    #[serde(default)]
+    pub fastapi: Option<FastapiSource>,
+
+    #[serde(default)]
+    pub django: Option<DjangoSource>,
+}
+
+
+#[derive(Debug, Deserialize)]
+pub struct FastapiSource {
+    pub module: String,
+
+    #[serde(default)]
+    pub cwd: Option<PathBuf>,
+
+    #[serde(default)]
+    pub python: Option<String>,
+
+    #[serde(default)]
+    pub command: Option<Vec<String>>,
+
+    #[serde(default)]
+    pub env: BTreeMap<String, String>,
+}
+
+
+#[derive(Debug, Deserialize)]
+pub struct DjangoSource {
+    pub manage_path: PathBuf,
+
+    #[serde(default)]
+    pub python: Option<String>,
+
+    #[serde(default)]
+    pub command: Option<Vec<String>>,
+
+    #[serde(default)]
+    pub env: BTreeMap<String, String>,
+}
+
+
+#[derive(Debug, Deserialize, Clone)]
+#[serde(untagged)]
+pub enum RustKernelSpec {
+    Path {
+        path: String,
+    },
+    Git {
+        git: String,
+        #[serde(default)]
+        tag: Option<String>,
+        #[serde(default)]
+        rev: Option<String>,
+        #[serde(default)]
+        branch: Option<String>,
+    },
+    Version {
+        version: String,
+    },
+}