mizan-axum + macros: state threading, array/map lowering, merge shape semantics

Three substrate extensions surfaced by the Blazr session port:

1. **App-state threading.** mizan-axum::router() is now generic over a
   user-supplied state type and threads `Arc<dyn Any + Send + Sync>` into
   every dispatch via RequestHandle. Handlers downcast to their concrete
   AppState. The stateless AFI fixture uses `router_stateless()` (matches
   the prior signature). RequestHandle gains a `from_dyn()` constructor
   to wrap already-erased trait-object references.

2. **`[T; N]` and `BTreeMap<K, V>` lowering in #[derive(Mizan)].** Fixed
   arrays emit as `List<T>` (matches Python `tuple[float,...]` → JSON
   array). String-keyed maps emit as `List<V>` — closest approximation
   until KDL grows a `dict` shape. Also: vec-element registrations get a
   per-function scope suffix so two handlers returning `Vec<Same>` don't
   collide at the static-name layer.

3. **`types_match` for merge: upsert-into-list semantics.** Now matches
   Python `types_match_for_merge`: direct (T == T), upsert (slot is
   `Alias(List(T))`, value is T), and list-replace (both sides list).
   The AFI fixture only exercised the direct path; the Blazr port's
   `morph_set_value` returning a single `MorphLayer` into a context with
   `Vec<MorphLayer>` slot is what surfaced the gap.

AFI codegen + wire parity stays 12/12 green after these substrate changes.

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

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

@@ -1,6 +1,6 @@
 //! HTTP handlers. Mirrors `backends/mizan-fastapi/src/mizan_fastapi/router.py`.
 
-use axum::extract::{Path, Query};
+use axum::extract::{Path, Query, State};
 use axum::http::{header, HeaderValue, StatusCode};
 use axum::response::{IntoResponse, Response};
 use axum::Json;
@@ -10,10 +10,17 @@ use mizan_core::{
 };
 use serde::{Deserialize, Serialize};
 use serde_json::{Map, Value};
+use std::any::Any;
 use std::collections::BTreeMap;
+use std::sync::Arc;
 
 use crate::errors::ApiError;
 
+/// Type-erased application state threaded into every `dispatch()` call via
+/// `RequestHandle`. User handlers downcast to their concrete state type.
+/// `Arc` keeps the clone cheap across per-request handler invocations.
+pub type AppStateAny = Arc<dyn Any + Send + Sync>;
+
 /// Body for POST /call/. Matches the Python `CallBody` shape.
 #[derive(Debug, Deserialize)]
 pub struct CallBody {
@@ -48,7 +55,10 @@ fn no_store(json: Value) -> Response {
 }
 
 /// POST /call/  — RPC dispatch.
-pub async fn function_call(Json(body): Json<CallBody>) -> Result<Response, ApiError> {
+pub async fn function_call(
+    State(app_state): State<AppStateAny>,
+    Json(body): Json<CallBody>,
+) -> Result<Response, ApiError> {
     let fn_name = body
         .resolved_name()
         .ok_or_else(|| ApiError(MizanError::BadRequest("missing `fn` field".into())))?
@@ -57,8 +67,7 @@ pub async fn function_call(Json(body): Json<CallBody>) -> Result<Response, ApiEr
     let fn_spec = lookup_function(&fn_name)
         .ok_or_else(|| ApiError(MizanError::NotFound(format!("function {fn_name:?} not registered"))))?;
 
-    let unit = ();
-    let req = RequestHandle::new(&unit);
+    let req = RequestHandle::from_dyn(app_state.as_ref());
     let result = fn_spec.dispatch(req, Value::Object(body.args.clone())).await.map_err(ApiError)?;
 
     let invalidate: Vec<Value> = compute_invalidation(fn_spec, &body.args)
@@ -82,6 +91,7 @@ pub async fn function_call(Json(body): Json<CallBody>) -> Result<Response, ApiEr
 
 /// GET /ctx/:context_name/  — bundled context fetch.
 pub async fn context_fetch(
+    State(app_state): State<AppStateAny>,
     Path(context_name): Path<String>,
     Query(params): Query<BTreeMap<String, String>>,
 ) -> Result<Response, ApiError> {
@@ -105,10 +115,9 @@ pub async fn context_fetch(
     // Convert query params (all-string values) to the JSON arg map. Numeric
     // params get parsed via the per-function input_params primitive table.
     let mut bundled = Map::new();
-    let unit = ();
     for fn_spec in &members {
         let args = coerce_query_args(*fn_spec, &params);
-        let req = RequestHandle::new(&unit);
+        let req = RequestHandle::from_dyn(app_state.as_ref());
         let result = fn_spec.dispatch(req, Value::Object(args)).await.map_err(ApiError)?;
         bundled.insert(fn_spec.name().to_string(), result);
     }

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

@@ -22,16 +22,37 @@ mod errors;
 mod handlers;
 
 pub use errors::ApiError;
-pub use handlers::{context_fetch, function_call, session_init, CallBody, CallResponse};
+pub use handlers::{
+    context_fetch, function_call, session_init, AppStateAny, CallBody, CallResponse,
+};
 
 use axum::routing::{get, post};
 use axum::Router;
+use std::any::Any;
+use std::sync::Arc;
 
-/// Build the Mizan router. Mount it under a prefix:
-///   `Router::new().nest("/api/mizan", router())`.
-pub fn router() -> Router {
+/// Build the Mizan router with user-supplied app state. The state is
+/// type-erased into an `Arc<dyn Any + Send + Sync>` and threaded into every
+/// dispatch via `RequestHandle`. Handlers downcast to their concrete state
+/// type.
+///
+/// Mount under a prefix:
+///   `Router::new().nest("/api/mizan", router(my_state))`.
+pub fn router<S>(state: S) -> Router
+where
+    S: Any + Send + Sync + 'static,
+{
+    let state: AppStateAny = Arc::new(state);
     Router::new()
         .route("/session/", get(handlers::session_init))
         .route("/call/", post(handlers::function_call))
         .route("/ctx/:context_name/", get(handlers::context_fetch))
+        .with_state(state)
+}
+
+/// Router variant for callers that have no app state to thread — the
+/// dispatch path receives a unit-typed handle. Used by the AFI fixture
+/// and other stateless test apps.
+pub fn router_stateless() -> Router {
+    router(())
 }