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/regen_fixture_client.py

@@ -0,0 +1,40 @@
+"""Regenerate the wire-parity fixture_client crate via the Rust codegen binary.
+
+Drives the Rust `mizan-generate` binary against `tests/rust/mizan.toml`,
+which points at the AFI fixture's FastAPI registration module. Output
+lands under `tests/rust/fixture_client/` and is consumed by both
+`drive_emitted` (typed-emitted-crate probes) and `drive_kernel`
+(raw-kernel probes) via the parent `Cargo.toml` workspace.
+"""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+from pathlib import Path
+
+
+HERE = Path(__file__).resolve().parent
+REPO_ROOT = HERE.parents[1]
+BINARY = REPO_ROOT / "protocol/mizan-codegen/target/release/mizan-generate"
+CONFIG = HERE / "mizan.toml"
+
+
+def main() -> int:
+    if not BINARY.exists():
+        sys.stderr.write(
+            f"[regen] binary missing: {BINARY}\n"
+            "[regen] build it: cargo build --release "
+            "--manifest-path protocol/mizan-codegen/Cargo.toml\n"
+        )
+        return 1
+
+    result = subprocess.run(
+        [str(BINARY), "--config", str(CONFIG)],
+        cwd=HERE,
+    )
+    return result.returncode
+
+
+if __name__ == "__main__":
+    sys.exit(main())