feat(character): restrict jumping to when on a platform

Change the jump() condition from checking velocity to detecting whether the character's feet are touching a platform. This prevents mid-air jumps and makes jump behavior more intuitive for platformer gameplay.

character.js

@@ -10,7 +10,20 @@ var c = {
     size: 50,
     img: null,
     jump() {
-        if ( this.velocity <= 0.15 ) {
+        const feet = this.pos.y + this.size;
+
+        const onPlatform = getPlatforms().some(p => {
+            const overlapsX =
+                this.pos.x + this.size > p.position.x &&
+                this.pos.x < p.position.x + p.size.x;
+
+            const touchesTop =
+                Math.abs(feet - p.position.y) < 2;
+
+            return overlapsX && touchesTop;
+        });
+
+        if (onPlatform) {
             this.velocity.y = -5;
         }
     },