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.

src/server/adapter.rs

@@ -855,7 +855,7 @@ mod tests {
     async fn openapi_json_serves_the_gateway_projection() {
         use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
-        let mut registry = OperationRegistry::new();
+        let registry = OperationRegistry::new();
         let spec = alkcall::registry::spec::OperationSpec::new(
             "echo/run",
             alkcall::registry::spec::OperationType::Query,