phase 3: port alknet-tty-local behind the local feature

Folds the alknet-tty-local crate into alktty as a feature-gated local
submodule (the single-crate + local-feature design from ADR-054; the
cyclic-dep workaround the mono-repo needed doesn't apply to one crate).

src/local/mod.rs:
- Module root, re-exports LocalTtyBackend, declares backend/pipe/pty
- Doc-commented as feature-gated + non-wasm by design (portable-pty +
  tokio::process need a real OS; enabling local on a wasm target is a
  build error by design)

src/local/backend.rs:
- LocalTtyBackend: implements crate::backend::TtyBackend
- allocate() dispatches on params.terminal: Some -> pty::allocate_pty,
  None -> pipe::allocate_pipe (ADR-054)
- 7 tests: PTY/pipe dispatch, empty-cmd rejection (both modes),
  resource_id returns None, new() constructs

src/local/pty.rs:
- portable_pty + 3 std threads (reader/writer/waiter) feeding tokio
  mpsc/oneshot (REQ-TTY-01 blocking->async bridge)
- PtyControl: resize + REQ-TTY-02 process-group signal forwarding
  (libc::kill(-pgid, sig) + kill(pid, sig) fallback + ChildKiller::kill
  for unknown names)
- LocalExitFuture: ADR-056 kill-on-Drop guard (ChildKiller on cancel,
  Option::take disarms on resolve)
- StdinSink: AsyncWrite over mpc::Sender<StdinCmd> with in-flight
  reserve+send parking for full-channel backpressure
- 7 tests incl. process-group reach (bash -c sleep), cancel-cleanup,
  unknown-signal fallback

src/local/pipe.rs:
- tokio::process::Command + tokio_util::io::ReaderStream
- PipeControl: no-op resize, libc::kill(pid, sig) with SIGKILL
  fallback for unknown names (pid-only, no process group - documented
  limitation of the runner case)
- PipeExitFuture: in-place Child::wait() poll (avoids self-referential
  borrow) + ADR-056 kill-on-Drop guard (start_kill on cancel)
- BytesStream: wraps ReaderStream, strips io::Error to EOF
- 9 tests incl. separate stderr, SIGTERM=-15, SIGKILL=-9 fallback,
  cancel-cleanup pid probe (kill(pid,0) returns ESRCH)

Import migration: alknet_tty::backend::{...} -> crate::backend::{...},
alknet_tty::control::signal_from_name -> crate::control::signal_from_name.

Cargo.toml: no changes needed (portable-pty + tokio-util optional, libc
under cfg(unix), and the local feature wiring tokio/process +
tokio/rt-multi-thread were already in the scaffold per Phase 0).

Also fixes pre-existing rustfmt drift in adapter.rs/channels.rs/
session.rs/lib.rs left by the phase 2 commit (cargo fmt --check without
--features local reported 28 diffs; cargo fmt does not accept
--features, so the earlier 'clean' check was a false negative — the
check errored on the unknown flag and grepped an empty stdout). Lesson:
run cargo fmt --check with no feature flags; cargo fmt doesn't gate on
features.

Verification:
- cargo test --features local -> 103/103 pass (was 80 at phase 2 end;
  +23 new tests across the 3 local modules: 7 backend, 7 pty, 9 pipe)
- cargo test (no features) -> 80/80 pass (local module not compiled)
- cargo clippy --features local --all-targets -> clean
- cargo clippy --all-targets (no features) -> clean
- cargo check --target wasm32-unknown-unknown -> clean (default crate
  stays wasm-clean; local is feature-gated and non-wasm by design)
- cargo fmt --check -> clean

src/adapter.rs

@@ -76,8 +76,7 @@ use tracing::{debug, warn};
 use crate::backend::{TtyBackend, TtyHandle};
 use crate::control::ControlMessage;
 use crate::negotiation::{
-    error_response_bytes, NegotiateRequest, NegotiationError, NegotiationReader,
-    NegotiationWriter,
+    error_response_bytes, NegotiateRequest, NegotiationError, NegotiationReader, NegotiationWriter,
 };
 use crate::wire::{Chunk, ChunkReader, ChunkWriter, RawError, STREAM_CTRL_IN, STREAM_STDIN};