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>

package-lock.json

@@ -8,6 +8,8 @@
       "name": "friends-holdem",
       "version": "0.1.0",
       "dependencies": {
+        "@pokertools/engine": "2.1.0",
+        "@pokertools/evaluator": "2.1.0",
         "express": "^4.22.3",
         "socket.io": "^4.8.4"
       },
@@ -15,6 +17,47 @@
         "node": ">=18"
       }
     },
+    "node_modules/@pokertools/engine": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/@pokertools/engine/-/engine-2.1.0.tgz",
+      "integrity": "sha512-/3kXKpFf6nHygWQuX1AMu8yQa6HW9RhLW7TkOZJnXSESJ/vs00YWs976F2zNzqnlBPK90LvRSw4ZyqwK+9Ajow==",
+      "license": "MIT",
+      "dependencies": {
+        "@pokertools/evaluator": "2.1.0",
+        "@pokertools/types": "2.1.0"
+      },
+      "engines": {
+        "node": "^24.15.0 || >=26.0.0",
+        "npm": ">=12.2.0"
+      },
+      "peerDependencies": {
+        "@pokertools/evaluator": "2.1.0",
+        "@pokertools/types": "2.1.0"
+      }
+    },
+    "node_modules/@pokertools/evaluator": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/@pokertools/evaluator/-/evaluator-2.1.0.tgz",
+      "integrity": "sha512-9ZmmwrA9g2IvKST2y0HrdiwTGIM6uoMoom9fXq0ZfQHGcd24X5ydQJi3S5ciR/LFjSN4T2EakwXqVED6tuWD+w==",
+      "license": "MIT",
+      "engines": {
+        "node": "^24.15.0 || >=26.0.0",
+        "npm": ">=12.2.0"
+      }
+    },
+    "node_modules/@pokertools/types": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/@pokertools/types/-/types-2.1.0.tgz",
+      "integrity": "sha512-FBKLd3cczFfHQAH4DY0n+VEcdccEW7C5wa4PaAA/jaBvmYo0OU7PBHduPEhcO3ZpWBAr3KjW1ESzaajhlvTzCw==",
+      "license": "MIT",
+      "dependencies": {
+        "zod": "^4.6.5"
+      },
+      "engines": {
+        "node": "^24.15.0 || >=26.0.0",
+        "npm": ">=12.2.0"
+      }
+    },
     "node_modules/@socket.io/component-emitter": {
       "version": "3.1.2",
       "resolved": "https://registry.npmjs.org/@socket.io/component-emitter/-/component-emitter-3.1.2.tgz",
@@ -1078,6 +1121,15 @@
           "optional": true
         }
       }
+    },
+    "node_modules/zod": {
+      "version": "4.6.7",
+      "resolved": "https://registry.npmjs.org/zod/-/zod-4.6.7.tgz",
+      "integrity": "sha512-g8W2D2kU6a8untc053fX1uVBKzM7yqAxW/3zwk83+6RyF2+SKOuV+c0wBDhUwzV1PaGapmZH+rmcUzPf+3CGwg==",
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/colinhacks"
+      }
     }
   }
 }