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/tests/python_parity.rs

@@ -0,0 +1,75 @@
+//! Byte-equivalence test for the Python target against the JS baseline.
+
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+
+use mizan_codegen::config::{Config, SourceConfig};
+use mizan_codegen::emit::{CodegenTarget, EmittedFile};
+use mizan_codegen::emit::python::PythonClient;
+use mizan_codegen::fetch::parse_ir_from_str;
+
+
+fn load_ir() -> mizan_codegen::ir::MizanIR {
+    let path = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/afi_schema.json");
+    parse_ir_from_str(&std::fs::read_to_string(&path).unwrap()).unwrap()
+}
+
+
+fn fixture_config() -> Config {
+    Config {
+        project_id: None,
+        output: PathBuf::from("/tmp"),
+        targets: vec!["python".to_string()],
+        source: SourceConfig { fastapi: None, django: None },
+        rust_kernel: None,
+        rust_crate_name: None,
+    }
+}
+
+
+fn read_baseline(rel: &str) -> String {
+    let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("tests/fixtures/js_python")
+        .join(rel);
+    std::fs::read_to_string(&path)
+        .unwrap_or_else(|e| panic!("read baseline {}: {e}", path.display()))
+}
+
+
+fn emit_index(files: &[EmittedFile]) -> BTreeMap<PathBuf, &str> {
+    files.iter().map(|f| (f.rel_path.clone(), f.content.as_str())).collect()
+}
+
+
+fn assert_byte_equal(rel: &str, files: &BTreeMap<PathBuf, &str>) {
+    let actual = files
+        .get(&PathBuf::from(rel))
+        .unwrap_or_else(|| panic!("Python target did not produce {rel}"));
+    let expected = read_baseline(rel);
+    if *actual != expected {
+        for (lineno, (a, b)) in actual.lines().zip(expected.lines()).enumerate() {
+            if a != b {
+                panic!(
+                    "{rel} diverges at line {}:\n  expected: {b:?}\n  actual:   {a:?}",
+                    lineno + 1,
+                );
+            }
+        }
+        panic!(
+            "{rel} diverges in length: actual={} expected={}",
+            actual.len(), expected.len(),
+        );
+    }
+}
+
+
+#[test]
+fn python_target_all_files_match_baseline() {
+    let ir = load_ir();
+    let files = PythonClient.emit(&ir, &fixture_config());
+    let index = emit_index(&files);
+
+    for rel in ["types.py", "client.py", "__init__.py"] {
+        assert_byte_equal(rel, &index);
+    }
+}