Add a live population HUD with a boom/bust sparkline

A glass readout in the corner shows the current organism count and an
auto-scaled sparkline of recent population, so the ecosystem's boom/bust
waves are finally legible at a glance and slider/cull/bloom effects show up
immediately. Sampled ~3x/sec; non-interactive so it never blocks the canvas.

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