// at top of Player.update():
this.prevX = this.x;
this.prevY = this.y;

// ... your movement math ...

// precise land-from-above collision
this.grounded = false;
const cand = getNearbyPlatforms(this.x, this.y); // uses the grid (added below)
const bottom = this.y + this.size / 2;
const prevBottom = this.prevY + this.size / 2;

for (const p of cand) {
  // horizontal overlap first
  const leftHit  = this.x + this.size/2 > p.x;
  const rightHit = this.x - this.size/2 < p.x + p.w;
  if (!leftHit || !rightHit) continue;

  // must be moving downward and crossing the top surface
  const crossingTop = this.vy >= 0 && prevBottom <= p.y && bottom >= p.y;
  if (!crossingTop) continue;

  // snap onto the top face
  this.y = p.y - this.size / 2;
  this.vy = 0;
  this.grounded = true;
}