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>

cores/mizan-rust/src/runtime.rs

@@ -17,10 +17,18 @@ pub struct RequestHandle<'a> {
 }
 
 impl<'a> RequestHandle<'a> {
+    /// Wrap a typed reference. The most common path — handlers downcast back
+    /// to `T` via `downcast::<T>()`.
     pub fn new<T: Any + Send + Sync>(req: &'a T) -> Self {
         Self { inner: req }
     }
 
+    /// Wrap an already-erased `dyn Any` reference. Used by HTTP adapters
+    /// that thread an `Arc<dyn Any + Send + Sync>` app state in.
+    pub fn from_dyn(req: &'a (dyn Any + Send + Sync)) -> Self {
+        Self { inner: req }
+    }
+
     pub fn downcast<T: Any + Send + Sync>(&self) -> Option<&'a T> {
         self.inner.downcast_ref::<T>()
     }