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

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

@@ -0,0 +1,8 @@
+//! `CallAdapter`: implements `ProtocolHandler` for ALPN `alknet/call`.
+//!
+//! Accepts bidirectional streams, reads `EventEnvelope` frames, and
+//! dispatches `call.requested` events to the operation registry. See
+//! `docs/architecture/crates/call/call-protocol.md` for the full
+//! specification.
+
+// TODO: implement

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

@@ -0,0 +1,8 @@
+//! `CallConnection`: an established `alknet/call` connection (either
+//! direction — accepted or opened). Holds the connection's Layer 2 overlay
+//! (imported ops).
+//!
+//! See `docs/architecture/crates/call/call-protocol.md` for the full
+//! specification.
+
+// TODO: implement

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

@@ -0,0 +1,11 @@
+//! Call protocol: wire format, streams, and the call adapter.
+//!
+//! Implements `ProtocolHandler` for ALPN `alknet/call` on top of the
+//! operation registry. See `docs/architecture/crates/call/call-protocol.md`
+//! for the full specification.
+
+pub mod abort;
+pub mod adapter;
+pub mod connection;
+pub mod pending;
+pub mod wire;

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

@@ -0,0 +1,7 @@
+//! Pending request tracking: `PendingRequestMap` and `PendingEntry`.
+//!
+//! Correlates `call.responded` events back to the original `call.requested`
+//! by request ID. See `docs/architecture/crates/call/call-protocol.md` for
+//! the full specification.
+
+// TODO: implement

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

@@ -0,0 +1,7 @@
+//! Wire format: `EventEnvelope`, `ResponseEnvelope`, `CallError`, and
+//! length-prefixed JSON framing.
+//!
+//! See `docs/architecture/crates/call/call-protocol.md` for the full
+//! specification.
+
+// TODO: implement