Shiny Pings

What it does

• Ping an entity → it gets a glow outline and a fake advancement toast pops in the
  top-right ("You spotted a Zombie") using vanilla's real advancement UI (smooth slide +
  sound), with the mob's representative drop as the icon (zombie → rotten flesh, creeper →
  gunpowder, …).
• The ping spreads → when you spot a mob, the spot jumps on its own to mobs within a
  10-block radius, and from each of those to their neighbours, wave by wave (with an amethyst
  sound at each hop). Each mob is spotted only once, so it never loops: if A → B → C and C
  is next to A, A is skipped because it was already marked.
• Ping a dropped item → a card appears at the top-centre of the screen: the item inside
  a dark slot (the F4 menu slot) with the distance to the item below it (updated
  live as you move). It lasts ~10 seconds, and the item on the ground also glows so you can
  find it in the world.

Both the item card and the mob toast show one at a time: if you ping something else before
the previous one fades, it's cut and the new one takes over (they don't stack).