Fix smash item physics radius to align visual bottom edge with collision boundary

The circle hitbox was using a radius of 22 * WORLD_SCALE, which caused gravity items to hover above the ground and other bodies due to a gap between their visual bottom edge and physical collision boundary. Reduced the radius to 16 * WORLD_SCALE (displaySize/4) so that the Matter.js body's on-screen radius of 64 world pixels matches the item's actual visual extent, ensuring items land flush with terrain and other objects instead of floating above them.

src/config.js

@@ -213,7 +213,16 @@ export const SCORE = {
 // which fly under a reduced fraction, see KID.ejectedGravityScale),
 // tumbling as it goes, then lands and stays where it lands.
 export const SMASH_ITEM = {
-  radius: 22 * WORLD_SCALE, // circle hitbox (~70% of the 64px frame's display half)
+  // Circle hitbox. The spritesheet art sits in the LOWER half of each 64x64
+  // cell and fills down to the very bottom of that cell, so an item's VISUAL
+  // bottom edge is exactly displayHalf (= SMASH_ITEM.displaySize / 2 = 64
+  // world px) below its centre. In this build a Matter circle's on-screen
+  // radius is 2x the value passed to shape.radius (32 -> 64 world px, the
+  // game's own convention), so we pass displaySize/4 to make the PHYSICAL
+  // bottom coincide with the VISUAL bottom. That is what stops a gravity item
+  // from hovering above the ground - its art actually reaches the terrain
+  // (and any other body it falls onto) instead of floating a gap above it.
+  radius: 16 * WORLD_SCALE, // = SMASH_ITEM.displaySize / 4 (body radius -> 64)
   displaySize: 64 * WORLD_SCALE, // one 64x64 frame, shown square
   density: 0.001,
   friction: 0.5,