Add Interstellar Expedition roadmap

[RRG314]

Summary

• define Free Flight, Assisted Travel, and optional Interstellar Expeditions as three travel depths through the same universe
• stage one integrated expedition program covering research decisions, planning, ships, crew, resources, strategic time, resupply, discovery, multiplayer, and persistent outposts
• add connected completion journeys, failure causality, conservation, persistence, desktop/mobile review, and a clear scope boundary

Validation

• npm run verify:source
• git diff --check

Documentation only. No runtime, hosting, backend, rules, credentials, or player data changed.