fix: reveal tossup answer when all remaining players have buzzed

[xpoes123]

Summary

quizbowl/TossupRoom.js decides whether to reveal the answer after a neg by checking this.buzzes.length === Object.keys(this.sockets).length. But this.buzzes is only reset between tossups (endCurrentTossup) — Room.leave() removes the userId from this.sockets and never touches this.buzzes. So once a player negs and leaves, the buzz array carries a stale entry forever, and the equality check drifts out of sync with the actual room population.

Failure scenario

1. Players A and B are in a room with rebuzz off.
2. A buzzes and negs → buzzes = [A], sockets = {A, B} (1 ≠ 2, reading resumes — correct).
3. A leaves → buzzes = [A] (stale), sockets = {B}.
4. B buzzes and negs → buzzes = [A, B], sockets = {B} (2 ≠ 1, answer is not revealed even though every player still in the room has already buzzed).

A symmetric variant (A negs, B leaves without buzzing) makes the answer reveal after a single neg, which is also wrong when rebuzz is off.

Fix

Replace the length comparison with "every current socket has already buzzed":

Object.keys(this.sockets).every(id => this.buzzes.includes(id))

This ignores ghost entries left behind by departed players and correctly accounts for newly joined players who haven't buzzed yet (they'll be in sockets but not in buzzes, so the answer won't prematurely reveal).

Test plan

• Two-player room, rebuzz off: player A buzzes/negs, leaves, player B buzzes/negs → answer is revealed.
• Two-player room, rebuzz off: player A buzzes/negs, player B joins mid-tossup → reading continues until B also buzzes or the question ends (no premature reveal).
• Single-player room, rebuzz off: solo player negs → answer revealed (unchanged behavior).
• rebuzz on: behavior unchanged (the whole branch is gated on !this.settings.rebuzz).

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