feat(tty): initialize alknet-tty crate skeleton

Create crates/alknet-tty with Cargo.toml (alknet-core path dep, tokio,
bytes, futures-core, tokio-stream, serde, serde_json, async-trait,
tracing, thiserror — no portable_pty/bollard/russh/alknet-call per
ADR-057), the `local` feature gate (dep wiring deferred to
tty/local-feature-reexport), src/lib.rs with the crate doc comment and
module declarations, and skeleton modules (wire, control, negotiation,
backend, adapter) with doc comments and TODO markers. Add the crate to
the workspace members list.

cargo check, clippy -D warnings, and fmt --check all pass.

crates/alknet-tty/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "alknet-tty"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Terminal session protocol handler for the `alknet/tty` ALPN: wire format, TtyBackend trait, and TtyAdapter (ProtocolHandler)"
+repository.workspace = true
+
+[lib]
+name = "alknet_tty"
+
+[features]
+default = []
+# `local` re-exports `alknet_tty_local::LocalTtyBackend`. The
+# `dep:alknet-tty-local` dependency is wired in task `tty/local-feature-reexport`.
+local = []
+
+[dependencies]
+alknet-core = { path = "../alknet-core" }
+tokio = { version = "1", features = ["full"] }
+bytes = "1"
+futures-core = "0.3"
+tokio-stream = "0.1"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+async-trait = "0.1"
+tracing = "0.1"
+thiserror = "2"

crates/alknet-tty/src/adapter.rs

@@ -0,0 +1,4 @@
+//! `TtyAdapter` (`ProtocolHandler` on `alknet/tty`) and the `drive_session`
+//! three-pump bidirectional driver.
+
+// TODO: implement

crates/alknet-tty/src/backend.rs

@@ -0,0 +1,4 @@
+//! Backend trait and handle shapes: `TtyBackend`, `TtyHandle`, `TtyControl`,
+//! `TtyControlHandle`, `TtyParams`, `TerminalParams`, and `TtyError`.
+
+// TODO: implement

crates/alknet-tty/src/control.rs

@@ -0,0 +1,4 @@
+//! Control channel messages: the `ControlMessage` tagged enum and
+//! `signal_from_name`.
+
+// TODO: implement

crates/alknet-tty/src/lib.rs

@@ -0,0 +1,11 @@
+//! alknet-tty: Terminal session protocol handler for the `alknet/tty` ALPN.
+//!
+//! Two-carriage model (ADR-052): a JSON negotiation frame, then raw chunks
+//! (`[stream_type: u8][length: u32 be][payload]`). Backend-agnostic via the
+//! `TtyBackend` trait (ADR-053). Depends on alknet-core only (ADR-057).
+
+pub mod adapter;
+pub mod backend;
+pub mod control;
+pub mod negotiation;
+pub mod wire;

crates/alknet-tty/src/negotiation.rs

@@ -0,0 +1,4 @@
+//! Negotiation carriage: `NegotiateRequest`, `TerminalParamsWire`,
+//! length-prefixed framing reader/writer, and the error response shape.
+
+// TODO: implement

crates/alknet-tty/src/wire.rs

@@ -0,0 +1,4 @@
+//! Raw chunk wire format: `ChunkReader`/`ChunkWriter`, `Chunk`, `RawError`,
+//! stream-type constants, and `MAX_CHUNK_LEN`.
+
+// TODO: implement