Adds a tiny zone-based profiler (prof.{h,cpp}) enabled via KOBO_PROF=1.
Instruments run_game with zones for myship.move, enemies.move (split into
realize and move_pass), hit_structure, and hit_bolt. Adds KOBO_STRESS=N
to keep the enemy pool topped up with rocks for stable per-frame timing.
Why: the entity/render pipeline refactor in BENCH.md needs a comparable
baseline before any layout changes. Without a stress workload the natural
gameplay enemy count is too noisy to compare across commits.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
