chore(deps): consume alkcall 0.3 — per-session fork, serving loop, op/register

Bump alkcall 0.2 -> 0.3 (published 2026-09-04; the remediation of
alkcall reviews 004-005, the upstream resolution of alkhttp review 003
WS-24/WS-25). Two source-break classes, both mechanical:

- take_call_connection now returns Option<Arc<CallConnection>>
  (was bare value) — drop the double-wrap in WssSession::connect.
- register/register_on/register_openable take &OperationRegistry
  (was &mut) — drop 51 stale `mut` bindings (clippy --fix), which is
  the interior-mutability change that makes the per-session fork
  surface possible.

No behavior change in this crate yet; the 0.3 mechanisms (fork,
serve_single_stream, op/register, install_bootstrap_discovery,
collision policy) are the composition surface for alkhttp review 003
Unit 2 (WS-20..22, WS-26 wiring) — not wired here yet.

Verification: cargo test 453 passed (default) / 575 passed
(all-features), clippy both sides -D warnings clean, fmt clean,
cargo doc --no-deps clean.

tests/full_surface.rs

@@ -65,7 +65,7 @@ fn provider_with(tokens: Vec<(&str, Identity)>) -> Arc<dyn IdentityProvider> {
 /// and echo-restricted variants), a streaming sub op, and the discovery
 /// ops the adapters need.
 fn local_registry() -> Arc<OperationRegistry> {
-    let mut inner = OperationRegistry::new();
+    let inner = OperationRegistry::new();
     inner
         .register(HandlerRegistration::new(
             OperationSpec::new(
@@ -119,7 +119,7 @@ fn local_registry() -> Arc<OperationRegistry> {
         .unwrap();
     let inner = Arc::new(inner);
 
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             services_list_spec(),
@@ -351,7 +351,7 @@ async fn from_openapi_import_then_gateway_call() {
     // a wire call to them is NOT_FOUND (ADR-015 §2). The assembly layer
     // composes them under an External facade. Verify Internal-not-callable
     // through the gateway, then compose the External facade and call that.
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     for b in bundles {
         registry.register(b).unwrap();
     }

tests/ws_overlay_ops.rs

@@ -58,7 +58,7 @@ fn echo_handler() -> alkcall::registry::registration::Handler {
 }
 
 fn echo_registry() -> Arc<alkcall::registry::registration::OperationRegistry> {
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             external_spec("echo/run", AccessControl::default()),
@@ -510,7 +510,7 @@ async fn disconnect_drops_session_cleanly_subsequent_reach_fails_cleanly() {
 }
 
 fn slow_and_echo_registry() -> Arc<OperationRegistry> {
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             external_spec("slow/op", AccessControl::default()),

tests/ws_upgrade_session.rs

@@ -30,7 +30,7 @@ fn echo_handler() -> Handler {
 }
 
 fn echo_registry() -> Arc<OperationRegistry> {
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             OperationSpec::new(
@@ -54,7 +54,7 @@ fn echo_registry() -> Arc<OperationRegistry> {
 }
 
 fn slow_and_echo_registry() -> Arc<OperationRegistry> {
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             OperationSpec::new(
@@ -100,7 +100,7 @@ fn slow_and_echo_registry() -> Arc<OperationRegistry> {
 }
 
 fn internal_registry() -> Arc<OperationRegistry> {
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             OperationSpec::new(
@@ -124,7 +124,7 @@ fn internal_registry() -> Arc<OperationRegistry> {
 }
 
 fn restricted_registry() -> Arc<OperationRegistry> {
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             OperationSpec::new(
@@ -151,12 +151,12 @@ fn restricted_registry() -> Arc<OperationRegistry> {
 }
 
 fn registry_with_services_list(inner_ops: Vec<HandlerRegistration>) -> Arc<OperationRegistry> {
-    let mut inner = OperationRegistry::new();
+    let inner = OperationRegistry::new();
     for op in inner_ops {
         inner.register(op).unwrap();
     }
     let inner = Arc::new(inner);
-    let mut registry = OperationRegistry::new();
+    let registry = OperationRegistry::new();
     registry
         .register(HandlerRegistration::new(
             services_list_spec(),
@@ -696,7 +696,7 @@ async fn ws_sessions_registry_tracks_and_aborts_live_sessions() {
 async fn session_cap_rejects_over_limit_with_503_and_frees_slots_on_end() {
     use alkhttp::server::HttpAdapter;
 
-    let mut registry_val = OperationRegistry::new();
+    let registry_val = OperationRegistry::new();
     registry_val
         .register(HandlerRegistration::new(
             OperationSpec::new(