feat(tty-local): initialize alknet-tty-local sibling crate (ADR-054)

Create crates/alknet-tty-local with Cargo.toml (alknet-tty workspace dep,
portable-pty, libc unix-only, tokio, bytes, futures-core, tracing,
thiserror), src/lib.rs with module declarations and LocalTtyBackend
re-export, and skeleton module files (pty, pipe, backend) with doc
comments and TODO markers. Add the crate to the workspace members list.

crates/alknet-tty-local/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "alknet-tty-local"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Local TTY backend for alknet-tty: LocalTtyBackend via portable_pty (PTY mode) and std::process::Command (pipe/runner mode)"
+repository.workspace = true
+
+[lib]
+name = "alknet_tty_local"
+
+[dependencies]
+alknet-tty = { path = "../alknet-tty" }
+portable-pty = "0.8"
+tokio = { version = "1", features = ["full"] }
+bytes = "1"
+futures-core = "0.3"
+tracing = "0.1"
+thiserror = "2"
+
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"

crates/alknet-tty-local/src/backend.rs

@@ -0,0 +1,16 @@
+//! `LocalTtyBackend`: the `TtyBackend` implementation for local processes.
+//!
+//! `allocate()` branches on `TtyParams::terminal`: `Some(TerminalParams)`
+//! selects PTY mode (this crate's `pty` module); `None` selects pipe mode
+//! (the runner case, this crate's `pipe` module). One backend serves both
+//! (ADR-054).
+
+// TODO: implement
+
+/// Local TTY backend: implements `alknet_tty::TtyBackend` for local processes.
+///
+/// `allocate()` branches on `TtyParams::terminal`: `Some(TerminalParams)`
+/// selects PTY mode (this crate's `pty` module); `None` selects pipe mode
+/// (the runner case, this crate's `pipe` module). One backend serves both
+/// (ADR-054).
+pub struct LocalTtyBackend;

crates/alknet-tty-local/src/lib.rs

@@ -0,0 +1,13 @@
+//! alknet-tty-local: Local TTY backend for alknet-tty.
+//!
+//! `LocalTtyBackend` implements `alknet_tty::TtyBackend` via `portable_pty`
+//! (PTY mode, terminal semantics) and `std::process::Command` (pipe mode,
+//! the runner case). The blocking→async bridge for PTY mode uses three
+//! dedicated std threads feeding tokio mpsc/oneshot channels (REQ-TTY-01).
+//! Signal forwarding targets the foreground process group (REQ-TTY-02).
+
+pub mod backend;
+pub mod pipe;
+pub mod pty;
+
+pub use backend::LocalTtyBackend;

crates/alknet-tty-local/src/pipe.rs

@@ -0,0 +1,8 @@
+//! Pipe mode for `LocalTtyBackend`: `tokio::process`-backed runner sessions.
+//!
+//! Spawns the command with `Stdio::piped()` for stdin/stdout/stderr, exposes
+//! tokio-native `AsyncRead`/`AsyncWrite` (no std-thread bridge needed),
+//! `PipeControl` (no-op resize, `libc::kill(pid, sig)` signal), and the
+//! ADR-056 kill guard on the `exit_code` future.
+
+// TODO: implement

crates/alknet-tty-local/src/pty.rs

@@ -0,0 +1,9 @@
+//! PTY mode for `LocalTtyBackend`: `portable_pty`-backed terminal sessions.
+//!
+//! Implements the blocking→async bridge (REQ-TTY-01) via three dedicated
+//! std threads (reader, writer, waiter) feeding tokio mpsc/oneshot channels,
+//! `PtyControl` for resize/signal, REQ-TTY-02 process-group signal
+//! forwarding (`libc::kill(-pgid, sig)`), and the ADR-056 kill guard on the
+//! `exit_code` future.
+
+// TODO: implement