Adds the RNG seam skeleton (Task 1 of M12): IRandomSource (NextUnit/NextSelf) and RandomSourceBridge.Range mirroring BattleManagerBase.StableRandom exactly (`(int)Math.Floor(val * unit)`). RngSeamTests pins the floor arithmetic (1 test, passing). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>