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>

cores/mizan-rust/src/registry.rs

@@ -0,0 +1,47 @@
+//! Compile-time-populated registries, distributed across the consuming crate's
+//! source via linkme. The proc macros emit `#[linkme::distributed_slice(...)]`
+//! statics that land here at link time.
+
+use crate::ir::NamedType;
+use crate::traits::FunctionSpec;
+use linkme::distributed_slice;
+
+/// One named-type registration. Emitted by `#[derive(Mizan)]`.
+pub struct TypeEntry {
+    pub name: &'static str,
+    pub shape_fn: fn() -> NamedType,
+}
+
+/// One context-marker registration. Emitted by `#[mizan::context]`.
+pub struct ContextEntry {
+    pub name: &'static str,
+}
+
+#[distributed_slice]
+pub static TYPES: [TypeEntry] = [..];
+
+#[distributed_slice]
+pub static CONTEXTS: [ContextEntry] = [..];
+
+#[distributed_slice]
+pub static FUNCTIONS: [&'static dyn FunctionSpec] = [..];
+
+/// Find a registered function by wire name. Used by the HTTP adapter.
+pub fn lookup_function(name: &str) -> Option<&'static dyn FunctionSpec> {
+    FUNCTIONS.iter().copied().find(|f| f.name() == name)
+}
+
+/// Find a registered context by name. Used by graph_check.
+pub fn lookup_context(name: &str) -> Option<&'static ContextEntry> {
+    CONTEXTS.iter().find(|c| c.name == name)
+}
+
+/// All functions that declare a given context as their `context` membership.
+/// Order matches `FUNCTIONS` iteration order — i.e., registration order.
+pub fn context_members(ctx_name: &str) -> Vec<&'static dyn FunctionSpec> {
+    FUNCTIONS
+        .iter()
+        .copied()
+        .filter(|f| f.context() == Some(ctx_name))
+        .collect()
+}