Reject local-only outbound close codes

[ShipItAndPray]

Summary

• raise LocalProtocolError for outbound local-only close codes like 1006 and 1015
• keep 1005 as the existing sentinel for sending an empty close frame, and handle raw integer 1005 consistently
• add focused regression coverage at both the frame protocol and connection layers

Validation

• baseline behavior on parent commit rewrote 1006/1015 to 1000
• PYTHONPATH=/Users/somepalli/codex/oss-prs-next/wsproto-baseline/src .venv/bin/python -m pytest /tmp/test_wsproto_issue_182.py -q
• PYTHONPATH=/Users/somepalli/codex/oss-prs-next/wsproto/src .venv/bin/python -m pytest /tmp/test_wsproto_issue_182.py -q
• .venv/bin/python -m pytest tests/test_frame_protocol.py -k "local_only_close_reason or no_status_rcvd_close_reason" -q
• .venv/bin/python -m pytest tests/test_connection.py -k "local_only_close_reason_rejected or close_whilst_closing or send_after_close or closure" -q
• .venv/bin/python -m pytest -q

Closes #182.