chore: add py.typed marker for PEP 561 (#9)

Change-Id: Ie9a13296ca41494607095623698e17fe461d9c11
Claude-Session-Id: b0a3e393-05a6-4967-ab25-0d2a6fd26558

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: jd