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]).

Cargo.lock

@@ -750,6 +750,13 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "pty-mock"
+version = "0.1.4"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "quote"
 version = "1.0.43"