New missions table (schema + DDL). Pure model in lib/game/missions.ts:
DELIVER / EXPLORE / FIND contracts generated from missionRng(seed, turn),
board always topped up to >=3 AVAILABLE after every action. save.ts wires
SCHEMA_VERSION 5 -> 6 with a v6 backfill step and completion hooks inside
jumpToSystem/explorePlanet (one atomic persist). Unit tests cover generation
determinism, top-up idempotency, backfill, accept validation and per-type
completion incl. reward accumulation.
