feat(call): initialize alknet-call crate skeleton (task: call/crate-init)

Create crates/alknet-call with Cargo.toml, lib.rs, and module skeletons
for the registry (spec, context, registration, env, discovery) and
protocol (wire, pending, connection, adapter, abort) subsystems. Add the
crate to the workspace members list. Depends on alknet-core (workspace
path), irpc (workspace dep), tokio, serde, serde_json, async-trait,
tracing, thiserror, uuid, and futures. Implements ProtocolHandler on
ALPN alknet/call per docs/architecture/crates/call.

crates/alknet-call/src/protocol/abort.rs

@@ -0,0 +1,7 @@
+//! Abort cascade logic for nested calls (ADR-016).
+//!
+//! When `call.aborted` arrives for a parent request, the protocol cascades
+//! the abort to all non-terminal descendants in the call tree. Default
+//! policy is `abort-dependents`; `continue-running` is an opt-in.
+
+// TODO: implement