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/channels_smoke.rs

@@ -0,0 +1,80 @@
+//! Smoke test for the channels target against a synthetic fixture.
+//! The JS channels.mjs runs types through `openapi-typescript` which the
+//! Rust codegen replaces with direct interface emission; byte-equivalence
+//! against the JS baseline is intentionally not the gate. Instead this
+//! test checks structural properties of the emitted output.
+
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+
+use mizan_codegen::config::{Config, SourceConfig};
+use mizan_codegen::emit::CodegenTarget;
+use mizan_codegen::emit::channels::ChannelsTarget;
+use mizan_codegen::fetch::parse_ir_from_str;
+
+
+fn fixture_config() -> Config {
+    Config {
+        project_id: None,
+        output: PathBuf::from("/tmp"),
+        targets: vec!["channels".to_string()],
+        source: SourceConfig { fastapi: None, django: None },
+        rust_kernel: None,
+        rust_crate_name: None,
+    }
+}
+
+
+#[test]
+fn channels_target_emits_expected_files() {
+    let raw = std::fs::read_to_string(
+        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/channels_schema.json"),
+    ).unwrap();
+    let ir = parse_ir_from_str(&raw).unwrap();
+
+    let files = ChannelsTarget.emit(&ir, &fixture_config());
+    assert_eq!(files.len(), 2, "channels target emits 2 files when channels present");
+
+    let by_path: BTreeMap<PathBuf, &str> =
+        files.iter().map(|f| (f.rel_path.clone(), f.content.as_str())).collect();
+
+    let ts = by_path.get(&PathBuf::from("channels.ts"))
+        .expect("channels.ts emitted");
+    for expected in [
+        "export interface ChatChannelParams",
+        "export interface ChatReactMessage",
+        "export interface ChatDjangoMessage",
+        "export interface NotificationsDjangoMessage",
+        "export const CHANNELS = {",
+        "chat: {",
+        "notifications: {",
+        "hasParams: true",
+        "hasParams: false",
+    ] {
+        assert!(ts.contains(expected), "channels.ts must contain {expected:?}");
+    }
+
+    let hooks = by_path.get(&PathBuf::from("channels.hooks.tsx"))
+        .expect("channels.hooks.tsx emitted");
+    for expected in [
+        "import { useChannel, type ChannelSubscription } from 'mizan/channels'",
+        "export function useChatChannel(params: ChatChannelParams)",
+        "export function useNotificationsChannel()",
+        "ChannelSubscription<ChatChannelParams, ChatDjangoMessage, ChatReactMessage>",
+        "ChannelSubscription<Record<string, never>, NotificationsDjangoMessage, never>",
+    ] {
+        assert!(hooks.contains(expected), "channels.hooks.tsx must contain {expected:?}");
+    }
+}
+
+
+#[test]
+fn channels_target_emits_nothing_when_empty() {
+    // AFI fixture has no channels — target should produce zero files.
+    let raw = std::fs::read_to_string(
+        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/afi_schema.json"),
+    ).unwrap();
+    let ir = parse_ir_from_str(&raw).unwrap();
+    let files = ChannelsTarget.emit(&ir, &fixture_config());
+    assert!(files.is_empty(), "no channels → no files");
+}