Port the POC's control channel schema (alknet-tty-poc/src/control.rs)
into crates/alknet-tty/src/control.rs: the ControlMessage tagged enum
(Resize/Signal/Eof/Exit) with snake_case type tag, to_json/from_slice
helpers, and the Unix-only signal_from_name mapping the 9 supported
signal names to libc numbers. Unknown type tags return a serde_json
error; the adapter (later task) ignores that error per the wire spec's
extensibility policy. Adds libc as a target.'cfg(unix)' dependency.
Refs: docs/architecture/crates/tty/tty-wire.md §"Control Channel"
Refs: docs/architecture/decisions/055-exit-code-on-control-chunk.md
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.
glm-5.2