enhancement: Add live sidebar scoreboard showing player rankings during race

Description

There is no sidebar scoreboard during a race. Players have no awareness of their position relative to others. A live ranking display (standard Minecraft sidebar objective) is essential for the Mario Kart-style competitive feel.

Acceptance Criteria

• A sidebar scoreboard is shown to all players during the Game phase
• Scoreboard updates every second (not every tick — 20 Hz sidebar writes cause client flicker)
• Displayed info per row: rank, player name, rings passed / total rings, cup points
• Scoreboard is cleared when the Game phase ends
• Works correctly with 2–8 players simultaneously

Technical Details

• Use Minestom's `Scoreboard` / `Sidebar` API
• Update driver: a 20-tick (1 s) interval in `GamePhase.tick()` or a dedicated `ScoreboardSystem`
• Sort players by `ScoreComponent.ringsHit` descending; tie-break by time of last ring pass
• Remove `ScoreDisplaySystem`'s actionbar write once this scoreboard is active (consolidate HUD)

Dependencies

• Scoring source-of-truth issue (#99) resolved
• Elytra flight and phase wiring complete

Estimate

M

Start with GamePhase.tick(), ScoreDisplaySystem, and the scoring source of truth from issue #99; then review Minestom's Scoreboard/Sidebar API. Implement the Game-phase sidebar with one-second updates, ranking and tie-break rules, and the listed per-player fields, while removing the actionbar write. Verify it clears at phase end and works for 2–8 players.