fix: remove watergrass inner corner tiles from grass layer

The watergrass inner corner tiles (rows 0-1) show mostly water with a
tiny grass patch — they're designed for water tiles at concave corners,
not for grass tiles. Removing them eliminates the reddish-brown artifacts
at diagonal water-grass boundaries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/src/scenes/GameScene.ts

@@ -181,11 +181,8 @@ export class GameScene extends Phaser.Scene {
       if (s) return 13;
       if (w) return 9;
       if (e) return 11;
-      // Inner corners (diagonal-only water) — small water notch in grass
-      if (se) return 0;
-      if (sw) return 1;
-      if (ne) return 3;
-      if (nw) return 4;
+      // Inner corners skipped — watergrass inner tiles are designed for water
+      // tiles (mostly water with small grass patch), not for grass tiles.
       return -1;
     };