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.toml

@@ -2,6 +2,7 @@
 resolver = "2"
 members = [
     "crates/claude-chill",
+    "crates/pty-mock",
 ]
 
 [workspace.package]