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/fixtures/channels_schema.json

@@ -0,0 +1,55 @@
+{
+  "x-mizan-channels": [
+    {
+      "name": "chat",
+      "pascalName": "Chat",
+      "hasParams": true,
+      "hasReactMessage": true,
+      "hasDjangoMessage": true,
+      "paramsType": "ChatChannelParams",
+      "reactMessageType": "ChatReactMessage",
+      "djangoMessageType": "ChatDjangoMessage"
+    },
+    {
+      "name": "notifications",
+      "pascalName": "Notifications",
+      "hasParams": false,
+      "hasReactMessage": false,
+      "hasDjangoMessage": true,
+      "djangoMessageType": "NotificationsDjangoMessage"
+    }
+  ],
+  "components": {
+    "schemas": {
+      "ChatChannelParams": {
+        "type": "object",
+        "properties": {
+          "room_id": { "type": "string" }
+        },
+        "required": ["room_id"]
+      },
+      "ChatReactMessage": {
+        "type": "object",
+        "properties": {
+          "text": { "type": "string" }
+        },
+        "required": ["text"]
+      },
+      "ChatDjangoMessage": {
+        "type": "object",
+        "properties": {
+          "text": { "type": "string" },
+          "from_user": { "type": "string" }
+        },
+        "required": ["text", "from_user"]
+      },
+      "NotificationsDjangoMessage": {
+        "type": "object",
+        "properties": {
+          "body": { "type": "string" }
+        },
+        "required": ["body"]
+      }
+    }
+  }
+}