docs: add WTF-to-Ship roadmap — 7 phases, multiplayer-first

Prioritized plan to go from 240 WTFs to all 4 games working in
multiplayer. Phase 0 locks down security, phases 1-4 bring each game
online, phases 5-7 polish UX/stability/theming.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>