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/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()),