fix: preserve Enter in PTY subprocesses

[D3f0]

Summary

• preserve carriage-return bytes while forwarding foreground PTY stdin
• restore terminal attributes and serialize concurrent PTY terminal ownership
• add a nested-PTY regression test covering Enter and terminal restoration
• replace a Rich-wrapping-sensitive package safety assertion with an observable filesystem assertion

Closes #141

Verification

• uv run pytest -q --disable-warnings (762 passed, 18 skipped)
• uv run pytest tests/original_invoke -q (101 passed, 14 skipped)
• ruff and ty checks passed