feat: add Patient Zero coordinator experiment

[christopherjnelson]

Summary

• add nullable scenario-owned Patient Zero designation with legacy null migration and World Setup controls
• provide bounded coordinator-only global awareness, engine-authoritative private Zero broadcasts, and Patient-Zero-endpoint direct replies
• add restrained World Lab observability, safe export telemetry, focused deterministic tests, and contract/documentation updates

Semantics

• Patient Zero remains a normal physical agent with one ordinary move/infect/capture/wait action and no numerical bonuses
• Zero directives are private, player-hidden, bounded, and advisory; invalid role/channel combinations do not mutate state
• all non-Zero agents retain local/alliance awareness and ordinary DMs remain range-limited
• sequential committed-event ordering remains in place; simultaneous ticks and capture succession are deferred

Validation

• pnpm validate
• pnpm test:e2e

Both passed locally on 2026-08-20.