Correct PreflopChart percentile docs

The KDoc claimed the worst hand is percentile 1.0, but percentiles mark the
START of each class's band, so the last class begins at (1326-12)/1326 ~ 0.991
and no real hand reaches 1.0. Documents that this is correct for gating, since a
class is admitted when its band opens inside the range. Also notes that the
unused key slots keep 1.0 as an unreachable sentinel.

Docs only; no behaviour change. 37 tests still pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

engine/src/commonMain/kotlin/com/jsjdesigns/poker/core/PreflopChart.kt

@@ -26,7 +26,10 @@ import kotlin.random.Random
  */
 object PreflopChart {
 
-    /** 169 entries keyed by [key]; value is percentile where 0.0 is the best hand. */
+    /**
+     * Indexed by [key]. Only 169 of the [TABLE_SIZE] slots are real hand classes;
+     * the rest stay at 1.0 as an unreachable sentinel.
+     */
     private val percentiles: DoubleArray by lazy { build() }
 
     /** Canonical slot for a starting hand: pairs, then suited, then offsuit. */
@@ -46,8 +49,14 @@ object PreflopChart {
     private const val TABLE_SIZE = 13 + 169 + 169
 
     /**
-     * Percentile of this starting hand, 0.0 (aces) to 1.0 (worst).
-     * A player who "plays the top 20%" enters when this is <= 0.20.
+     * The share of dealt hands ranked strictly stronger than this one, so a
+     * player who "plays the top 20%" enters when this is `<= 0.20`.
+     *
+     * This is the **start** of the class's band, not its end: aces are 0.0, and
+     * the worst class (72o) begins at `(1326 - 12) / 1326` ≈ 0.991 because its own
+     * 12 combinations occupy the remainder. The value therefore never reaches 1.0
+     * for a real hand — which is correct for gating, since a class is admitted
+     * when its band starts inside the range.
      */
     fun percentile(hole: IntArray): Double = percentiles[key(hole)]