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.

Cargo.toml

@@ -42,7 +42,7 @@ h2 = ["server", "dep:hyper", "hyper-util/http2", "hyper/http2"]
 http1 = ["server", "dep:hyper", "hyper-util/http1", "hyper/http1"]
 
 [dependencies]
-alkcall = { version = "0.2", features = ["gateway"] }
+alkcall = { version = "0.3", features = ["gateway"] }
 arc-swap = { version = "1", optional = true }
 axum = { version = "0.8", optional = true, features = ["ws"] }
 bytes = "1"