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/templates/rust/Cargo.toml.j2

@@ -0,0 +1,11 @@
+[package]
+name = "{{ crate_name }}"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+mizan-rust = {{ kernel_dep }}
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["rt", "macros"] }
+

protocol/mizan-codegen/templates/rust/call.rs.j2

@@ -0,0 +1,17 @@
+// AUTO-GENERATED by mizan — do not edit
+
+use serde_json::Value;
+
+use mizan_rust::{MizanClient, MizanError};
+
+{% if !type_imports.is_empty() -%}
+use crate::types::{ {{- type_imports|join(", ") -}} };
+
+{% endif -%}
+pub async fn call_{{ snake }}(client: &MizanClient{{ input_param }}) -> Result<{{ return_type }}, MizanError> {
+    let args_value = {{ args_value }};
+    let raw = client.call("{{ name }}", args_value).await?;
+    serde_json::from_value(raw)
+        .map_err(|e| MizanError::transport(format!("decode {{ name }} result: {e}")))
+}
+

protocol/mizan-codegen/templates/rust/context.rs.j2

@@ -0,0 +1,37 @@
+// AUTO-GENERATED by mizan — do not edit
+
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+use mizan_rust::{MizanClient, MizanError};
+
+{% if !type_imports.is_empty() -%}
+use crate::types::{ {{- type_imports|join(", ") -}} };
+
+{% endif -%}
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct {{ pascal }}ContextData {
+{% for field in data_fields -%}
+{% if field.has_rename %}    #[serde(rename = "{{ field.raw_name }}")]
+{% endif %}    pub {{ field.ident }}: {{ field.ty }},
+{% endfor -%}
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct {{ pascal }}ContextParams {
+{% for p in params -%}
+{% if p.has_rename %}    #[serde(rename = "{{ p.raw_name }}")]
+{% endif %}    pub {{ p.ident }}: {{ p.ty }},
+{% endfor -%}
+}
+
+pub async fn fetch_{{ snake }}_context(
+    client: &MizanClient,
+    params: &{{ pascal }}ContextParams,
+) -> Result<{{ pascal }}ContextData, MizanError> {
+    let params_value = serde_json::to_value(params).unwrap_or(Value::Object(Default::default()));
+    let raw = client.fetch_context("{{ ctx_name }}", &params_value).await?;
+    serde_json::from_value(raw)
+        .map_err(|e| MizanError::transport(format!("decode {{ ctx_name }} context: {e}")))
+}
+

protocol/mizan-codegen/templates/rust/lib.rs.j2

@@ -0,0 +1,9 @@
+// AUTO-GENERATED by mizan — do not edit
+
+pub mod types;
+{% if has_contexts %}pub mod contexts;
+{% endif %}{% if has_mutations %}pub mod mutations;
+{% endif %}{% if has_functions %}pub mod functions;
+{% endif %}
+pub use mizan_rust::{MizanClient, MizanConfig, MizanError};
+

protocol/mizan-codegen/templates/rust/mod.rs.j2

@@ -0,0 +1,5 @@
+// AUTO-GENERATED by mizan — do not edit
+
+{% for name in modules -%}
+pub mod {{ name }};
+{% endfor %}

protocol/mizan-codegen/templates/rust/types.rs.j2

@@ -0,0 +1,8 @@
+// AUTO-GENERATED by mizan — do not edit
+
+#![allow(non_camel_case_types)]
+
+use serde::{Deserialize, Serialize};
+
+{{ schemas_block }}
+{{ hoisted_enums_block }}