Add event moments: predation flashes and cull/bloom shockwaves

A lightweight cosmetic effects layer draws expanding glowing rings into the
HDR scene: hot-gold flashes at predation kill sites (subsampled so a busy
food web sparkles rather than strobes), a green seed-burst on bloom and on
each click-to-seed, and a red shockwave on cull. Effects are sim-side
side-data (capped, aged per tick, never read back so determinism holds) with
their own additive pipeline; the ring is interpolated between ticks for a
smooth 60fps expansion. Birth/death already read via the health-driven fade.

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