Replace state machine and event bus doc stubs with full documentation

Replace the "Coming Soon" placeholder pages with comprehensive documentation
based on the now-implemented code. State machine docs cover BuyerDealStatus
(12 states), BuyerCampaignStatus (9 states), transition rules, guard conditions,
audit trail, Linear TV extensions, DealStore integration, and Mermaid diagrams.
Event bus docs cover EventType (13 types), Event model, InMemoryEventBus,
emit helpers (fail-open pattern), SQLite persistence, API endpoints, and
usage examples.

bead: buyer-5e9

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: atc964