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/svelte/svelte.ts.j2

@@ -0,0 +1,31 @@
+// AUTO-GENERATED by mizan — do not edit
+
+import { readable, type Readable } from 'svelte/store'
+import { registerContext, type ContextState } from '@mizan/base'
+
+{% if !stage1_imports.is_empty() -%}
+import { {{ stage1_imports|join(", ") }} } from '../index'
+
+{% endif -%}
+{% for ctx in contexts -%}
+export function create{{ ctx.pascal }}Context({% if ctx.has_params %}params: {{ ctx.pascal }}ContextParams{% endif %}) {
+    const store = readable<ContextState<{{ ctx.pascal }}ContextData>>(
+        { data: null, status: 'idle', error: null },
+        (set) => {
+            const handle = registerContext('{{ ctx.name }}', {{ ctx.params_arg }}, () => fetch{{ ctx.pascal }}Context({{ ctx.params_arg }}))
+            const unsub = handle.subscribe(() => set(handle.getState()))
+            handle.refetch()
+            return () => { unsub(); handle.unregister() }
+        },
+    )
+
+    return store
+}
+
+{% endfor -%}
+{% for call in call_exports -%}
+export { call{{ call }} } from '../index'
+{% endfor %}
+export type { ContextState } from '@mizan/base'
+export { configure, initSession, MizanError } from '@mizan/base'
+