Docs: AudiencePlan wire-format spec (ar-g7nf) (#101)

Add the missing docs/api/audience_plan_wire_format.md that buyer and
seller code already reference by section number in comments. Derived
entirely from the existing implementation in deals_client.py,
audience_plan.py, openrtb_builder.py, seller main.py, and
audience_plan_validator.py — no behavior invented.

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

Author: atc964