Mizan-Rust backend adapter: server-side substrate + three-way parity

Adds first-class Rust-backed Mizan to sit alongside mizan-django and
mizan-fastapi. A Rust dev writes:

    #[derive(Mizan, Serialize, Deserialize)]
    pub struct ProfileOutput { pub user_id: i64, pub name: String }

    #[mizan::context("user")]
    pub struct UserCtx;

    #[mizan::client(context = UserCtx)]
    pub async fn user_profile(_req: &RequestHandle<'_>, user_id: i64) -> ProfileOutput { ... }

…and gets byte-identical KDL to the Python emitters, served over the
same wire protocol the React / Rust / Vue / Svelte kernels speak.

New crates:
- cores/mizan-rust/         (Cargo: mizan-core)     — IR types, KDL emitter, traits, registry,
                                                       runtime (compute_invalidation / compute_merges
                                                       ported from mizan-fastapi), graph_check with
                                                       structural type-matching
- cores/mizan-rust-macros/  (Cargo: mizan-macros)   — #[derive(Mizan)], #[mizan::context],
                                                       #[mizan::client] proc macros
- backends/mizan-rust-axum/ (Cargo: mizan-axum)     — axum HTTP adapter: /session/, /call/, /ctx/:name/
- tests/afi/rust_app/                                — AFI fixture port + server / export-ir binaries

Substrate-shape moves required by cross-language equivalence:
- IR canonicalization: functions / contexts / context-members / shared-by
  now sort alphabetically in both Python and Rust emitters. The IR is a
  contract; linkme doesn't preserve declaration order, so canonical sort
  is the only stable mapping. afi_ir.kdl + per-target baselines regenerated.
- MizanType::TYPE_NAME is a const (with a default type_name() reader) so
  it's usable in linkme TypeEntry static initializers.
- Tree-shaken type registry: #[derive(Mizan)] only emits the trait impl;
  the #[mizan::client] macro registers canonical-named entries from
  fn signatures, including Vec<T> element types for ref resolution.
- Merge resolution is structural (NamedType shape comparison) rather than
  by name — matches the Python types_match_for_merge semantics.

Three-way forcing functions:
- tests/afi/test_codegen_parity.py — Django ≡ FastAPI ≡ Rust on KDL bytes (3 pass)
- tests/rust/run_wire_parity.py    — 12/12 probes against FastAPI + Rust (EXIT=0)

Incidental fixes surfaced by the new tests:
- Stale `from .registry import validate_registry` import removed from
  mizan-django/setup/discovery.py (referenced a function that no longer
  exists; was masking codegen-parity).
- BASE_DIR added to tests/afi/django_app/project/settings.py.
- /session/ endpoint added to mizan-fastapi for protocol-shaped readiness
  probe parity (wire-parity harness now polls /api/mizan/session/ on both
  backends rather than FastAPI's /openapi.json).
- Root .gitignore picks up Rust target/ across the tree so new crates
  don't need per-crate gitignore.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backends/mizan-rust-axum/src/lib.rs

@@ -0,0 +1,37 @@
+//! Mizan axum HTTP adapter — typed RPC over `mizan-core`'s function registry.
+//!
+//! Usage:
+//!   ```ignore
+//!   use axum::Router;
+//!   use mizan_axum::router;
+//!
+//!   #[tokio::main]
+//!   async fn main() {
+//!       let app = Router::new().nest("/api/mizan", router());
+//!       let listener = tokio::net::TcpListener::bind("127.0.0.1:8000").await.unwrap();
+//!       axum::serve(listener, app).await.unwrap();
+//!   }
+//!   ```
+//!
+//! Exposed endpoints (mirroring `mizan-fastapi` / `mizan-django`):
+//!   * `GET  /session/`  — session-init probe (placeholder CSRF token)
+//!   * `POST /call/`     — RPC dispatch with invalidate+merge response
+//!   * `GET  /ctx/:name/` — bundled context fetch
+
+mod errors;
+mod handlers;
+
+pub use errors::ApiError;
+pub use handlers::{context_fetch, function_call, session_init, CallBody, CallResponse};
+
+use axum::routing::{get, post};
+use axum::Router;
+
+/// Build the Mizan router. Mount it under a prefix:
+///   `Router::new().nest("/api/mizan", router())`.
+pub fn router() -> Router {
+    Router::new()
+        .route("/session/", get(handlers::session_init))
+        .route("/call/", post(handlers::function_call))
+        .route("/ctx/:context_name/", get(handlers::context_fetch))
+}