Use @pokertools/engine and evaluator for poker rules and hand ranking

Replace the hand-rolled betting rules, side pots, showdown and hand
evaluator with the published @pokertools/engine and @pokertools/evaluator
libraries (MIT, pinned to 2.1.0).

- server/hand.js now wraps a PokerEngine per hand: it maps seats,
  mirrors stacks, staggers the all-in runout, and pays host-chosen
  winners on cash-only tables.
- server/cards.js and server/ai.js use the library evaluator.
- Out-of-turn folds go through the engine's stand; all-in hands stay
  live when the player leaves; uncalled bets return to a leaver.
- The engine refuses some check-able folds that would leave a side pot
  without eligible players; those now return a message (or check for
  a leaving player) instead of crashing.
- BB must be greater than SB (engine requirement); Node >= 24.
- Tests rig cards via engine snapshots and cover the new edge cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/index.js

@@ -123,7 +123,7 @@ app.post('/api/tables', (req, res) => {
   if (tables.nameTaken(name)) return res.status(409).json({ error: '同じ名前のテーブルがすでにあります' });
   if (!intIn(startingStack, 10, 100000000)) return res.status(400).json({ error: '開始スタックが不正です' });
   if (!intIn(smallBlind, 1, startingStack)) return res.status(400).json({ error: 'SBが不正です' });
-  if (!intIn(bigBlind, smallBlind, startingStack)) return res.status(400).json({ error: 'BBはSB以上・開始スタック以下にしてください' });
+  if (!intIn(bigBlind, smallBlind + 1, startingStack)) return res.status(400).json({ error: 'BBはSBより大きく・開始スタック以下にしてください' });
   if (blindTimerEnabled && !intIn(blindIntervalMin, 1, 240)) return res.status(400).json({ error: 'インターバルは1〜240分で入力してください' });
   if (!MODES.includes(mode)) return res.status(400).json({ error: 'テーブルの種類が不正です' });
   if (mode === 'cpu' && !intIn(cpuCount, 1, config.MAX_SEATS - 1)) {