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>

frontends/mizan-rust/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "mizan-rust"
+version = "0.1.0"
+edition = "2021"
+description = "Mizan client kernel — Rust port of @mizan/base. Context registry, fetch/call, merge, invalidation, error envelope parsing. Same wire as the TS / Vue / Svelte clients."
+license = "MIT"
+
+[features]
+default = []
+pyo3 = ["dep:pyo3", "dep:pythonize"]
+
+[dependencies]
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "time", "io-util", "io-std"] }
+tokio-util = "0.7"
+reqwest = { version = "0.12", default-features = false, features = ["json", "cookies", "rustls-tls"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+serde_urlencoded = "0.7"
+
+pyo3 = { version = "0.22", optional = true, features = ["extension-module", "abi3-py311"] }
+pythonize = { version = "0.22", optional = true }
+
+[dev-dependencies]
+tokio = { version = "1", features = ["full"] }