feat(terminal): add opt-in flow control

.tegami/add-opt-in-flow-control.md

@@ -0,0 +1,11 @@
+---
+packages:
+  et:
+    type: patch
+---
+
+## Add opt-in terminal flow control
+
+Clients can now select lossless backpressure or oldest-output discard when
+terminal output outruns the network, keeping Ctrl-C and prompt responses
+bounded without changing the default session behavior.

crates/et-bin/Cargo.toml

@@ -39,6 +39,9 @@ signal-hook = "0.3.18"
 # Ctrl+C / console shutdown signalling, the Windows analogue of SIGINT/SIGTERM.
 ctrlc = "3.5"
 
+[dev-dependencies]
+socket2 = "0.6.1"
+
 [target.'cfg(target_os = "linux")'.dev-dependencies]
 nix = { version = "0.31.3", default-features = false, features = ["fs", "poll"] }
 rustix = { version = "1.1.4", features = ["process"] }

crates/et-bin/src/client.rs

@@ -266,6 +266,7 @@ fn run_client(
             command: args.command.as_deref(),
             no_exit: args.no_exit,
             keepalive: args.keepalive,
+            flow_control: args.flow_control,
             terminal_enabled: !args.no_terminal,
             lines: crate::client_terminal::RemoteLines::from(remote_mode.terminal_shell),
             connection_name: &request.host_alias,

crates/et-bin/src/client_environment.rs

@@ -189,6 +189,7 @@ mod tests {
             jumphost: Some(false),
             reversetunnels: vec![request; 128],
             environmentvariables: Default::default(),
+            flowcontrol: None,
         };
         let mut locale = vec![
             ("LC_ALL".to_owned(), "C".to_owned()),