test(tty): end-to-end integration tests for LocalTtyBackend + drive_session

Add 18 integration tests in crates/alknet-tty-local/tests/ exercising
the full stack (LocalTtyBackend + TtyAdapter::drive_session over
tokio::io::duplex, real commands), validating the two crates work
together through the TtyBackend trait seam.

Tests live in alknet-tty-local (not alknet-tty) per the feature-gate
deviation: the cyclic dependency alknet-tty → alknet-tty-local →
alknet-tty prevents alknet-tty from depending on alknet-tty-local, so
the integration tests live in the crate that depends on both. Unix-only
tests (signal, process-group) use #[cfg(unix)].

PTY mode (8): happy path echo, interactive cat round-trip, resize,
SIGINT signal, process-group signal (REQ-TTY-02), stdin EOF sentinel,
cancel cleanup (ADR-056), exit-chunk-is-last (ADR-055).

Pipe mode (6): happy path echo, separate stderr, SIGTERM signal,
cancel cleanup, resize no-op, stdout chunk + sentinel.

Negotiation errors (5): unknown_backend, malformed_negotiation (bad
JSON, carriage != raw, empty cmd), allocate_failed (nonexistent
binary).

A shared test harness (tests/common/mod.rs) provides the client-side
wire protocol helpers: write_negotiation, write_chunk, write_control,
try_read_chunk, read_chunk_timeout, read_error_frame (asserts the
0x00 first-byte framing-disambiguation invariant), read_until_exit,
assert_no_more_chunks, close_write_half, plus spawn_session which
wires drive_session over a duplex pair with a test identity carrying
tty:open.

Verification: cargo test -p alknet-tty-local (42 tests pass), cargo
clippy -p alknet-tty-local -- -D warnings (clean), cargo fmt --check
-p alknet-tty-local (clean).

crates/alknet-tty-local/Cargo.toml

@@ -25,7 +25,10 @@ thiserror = "2"
 libc = "0.2"
 
 [dev-dependencies]
+alknet-core = { path = "../alknet-core" }
+alknet-tty = { path = "../alknet-tty" }
 tokio = { version = "1", features = ["full", "test-util", "macros"] }
 tokio-stream = "0.1"
 serde_json = "1"
-tempfile = "3"
+tempfile = "3"
+bytes = "1"