test: add PTY integration test harness

Add a pty-mock workspace crate (echo/alt-screen/paste-echo/buffer-fill/
sync-blocks subcommands) plus an in-process integration test suite that
drives Proxy directly via a new_for_test constructor and #[cfg(test)]
state accessors. Sets up the regression net needed before swapping the
underlying VT emulator from vt100 to termwiz.

Pulled from upstream draft PR #35; the alt-screen feed-order semantic
change is intentionally deferred (its dedicated test is left #[ignore]).

crates/pty-mock/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "pty-mock"
+version.workspace = true
+edition.workspace = true
+authors.workspace = true
+
+[[bin]]
+name = "pty-mock"
+path = "src/bin.rs"
+
+[dependencies]
+clap = { version = "4", features = ["derive"] }