Let users click the canvas to seed life at the cursor

A left click (drag still pans, distinguished by a movement threshold) spawns
a tight burst of fresh creatures at the cursor via a new bloom_at — direct,
delightful play with the world. The JS inverts the world→screen transform to
place the burst, and a canvas tooltip surfaces the controls. Covered by a
test that the burst clusters at the click point.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>