feat(tty-local): implement PTY mode — three-thread bridge, PtyControl, REQ-TTY-02, ADR-056 kill guard

Implement allocate_pty in src/pty.rs: portable_pty openpty + spawn as
session leader (set_controlling_tty(true)), with the three-thread
blocking→async bridge (REQ-TTY-01): reader → mpsc<Bytes> with
zero-length EOF sentinel, writer draining mpsc<StdinCmd> (Bytes/Eof),
waiter → oneshot<i32>. TtyHandle.stdout is ReceiverStream<Bytes>,
stdin is a StdinSink AsyncWrite wrapper over mpsc::Sender<StdinCmd>
(parks a reserve+send future when the channel is full), stderr is None
(PTY merges), exit_code is LocalExitFuture.

PtyControl implements TtyControl: resize via MasterPty::resize; signal
via libc::kill(-pgid, sig) with kill(pid, sig) fallback (Unix, REQ-TTY-02)
using alknet_tty::signal_from_name; unknown names and non-Unix fall back
to ChildKiller::kill (SIGHUP).

LocalExitFuture wraps the oneshot::Receiver<i32> + a ChildKiller kill
guard (ADR-056). poll delegates to the receiver and disarms the guard
on Ready (no-op Drop on happy path); Drop on cancel calls
ChildKiller::kill (SIGHUP) — best-effort; the waiter thread reaps.

Bump portable-pty 0.8 → 0.9 to match the POC API (MasterPty: Send).
Add tokio-stream, serde_json (dev), tempfile (dev) deps.

Tests: happy path (echo), stdin round-trip (cat), resize, signal INT
kills child, signal reaches process group (bash -c "sleep 60"),
cancel cleanup on drop, unknown signal falls back to ChildKiller.

crates/alknet-tty-local/Cargo.toml

@@ -11,12 +11,19 @@ name = "alknet_tty_local"
 
 [dependencies]
 alknet-tty = { path = "../alknet-tty" }
-portable-pty = "0.8"
+portable-pty = "0.9"
 tokio = { version = "1", features = ["full"] }
+tokio-stream = "0.1"
 bytes = "1"
 futures-core = "0.3"
 tracing = "0.1"
 thiserror = "2"
 
 [target.'cfg(unix)'.dependencies]
-libc = "0.2"
+libc = "0.2"
+
+[dev-dependencies]
+tokio = { version = "1", features = ["full", "test-util", "macros"] }
+tokio-stream = "0.1"
+serde_json = "1"
+tempfile = "3"