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
glm-5.2
8b8de61186