fix: handle macOS PTY stdin buffer overflow

[D3f0]

Summary

• avoid macOS Python 3.14 FIONREAD buffer overflow while forwarding PTY stdin
• preserve readiness and fallback behavior, including zero-byte ioctl results
• add a real PTY regression test for escape-sequence input (ESC[A)

Verification

• uv run pytest -q -o addopts= — 730 passed, 18 skipped\n- focused PTY/runner tests — 19 passed\n- pre-commit hooks — passed\n\nCloses macOS PTY stdin forwarding raises SystemError buffer overflow on key input #136