use extension sets in word generation

AND left_extension_strip in play_right and right_extension_strip
in play_left before iterating KWG children. This prunes tiles
that pass the cross set but cannot extend toward board tiles
the traversal hasn't reached yet, avoiding unnecessary KWG arc
iterations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andy-k

src/movegen.rs

@@ -1523,6 +1523,8 @@ struct GenPlaceMovesParams<'a, CallbackType: FnMut(i8, &[u8], i32, f32), N: kwg:
     board_snapshot: &'a BoardSnapshot<'a, N, L>,
     board_strip: &'a [u8],
     cross_set_strip: &'a [CrossSet],
+    left_extension_strip: &'a [u64],
+    right_extension_strip: &'a [u64],
     remaining_word_multipliers_strip: &'a [i8],
     remaining_tile_multipliers_strip: &'a [i8],
     face_value_scores_strip: &'a [i8],
@@ -1655,6 +1657,11 @@ fn gen_classic_place_moves<
                 this_cross_bits = !1;
                 is_unique = true;
             };
+            // Filter by left extension set (tiles to the right the traversal hasn't seen).
+            this_cross_bits &= env.params.left_extension_strip[idx as usize];
+            if this_cross_bits == 0 {
+                return;
+            }
             let new_word_multiplier = acc.word_multiplier
                 * env.params.remaining_word_multipliers_strip[idx as usize] as i32;
             let tile_multiplier = env.params.remaining_tile_multipliers_strip[idx as usize];
@@ -1777,6 +1784,11 @@ fn gen_classic_place_moves<
                 this_cross_bits = !1;
                 is_unique = true;
             }
+            // Filter by right extension set (tiles to the left the traversal hasn't seen).
+            this_cross_bits &= env.params.right_extension_strip[idx as usize];
+            if this_cross_bits == 0 {
+                return;
+            }
             let new_word_multiplier = acc.word_multiplier
                 * env.params.remaining_word_multipliers_strip[idx as usize] as i32;
             let tile_multiplier = env.params.remaining_tile_multipliers_strip[idx as usize];
@@ -2234,6 +2246,20 @@ fn gen_place_moves_at<
             } else {
                 &working_buffer.cross_set_for_across_plays[strip_range_start..strip_range_end]
             },
+            left_extension_strip: if placement.down {
+                &working_buffer.left_extension_set_for_down_plays
+                    [strip_range_start..strip_range_end]
+            } else {
+                &working_buffer.left_extension_set_for_across_plays
+                    [strip_range_start..strip_range_end]
+            },
+            right_extension_strip: if placement.down {
+                &working_buffer.right_extension_set_for_down_plays
+                    [strip_range_start..strip_range_end]
+            } else {
+                &working_buffer.right_extension_set_for_across_plays
+                    [strip_range_start..strip_range_end]
+            },
             remaining_word_multipliers_strip: if placement.down {
                 &working_buffer.remaining_word_multipliers_for_down_plays
                     [strip_range_start..strip_range_end]