Flashlight/darkness render path drops FPS until battery dies

[jeffdarcy]

Observed during public GitHub Pages testing on a PC: frame rate was around 30 FPS while the game was running with the flashlight active, then jumped to about 120 FPS when the flashlight died.

Likely cause:

• The flashlight battery reaching zero disables the expensive lighting path: DarknessSystem.stampLight() returns early, renderBeamFx() returns early, and the active reveal/beam work decays.
• While the light is active, each frame pushes multiple reveal stamps, caps/maintains up to 96 stamps, redraws soft erase circles with 14 rings each, erases them from a screen-sized render texture, and draws beam haze/vignette graphics.
• On a 1080p / high-refresh monitor this can become fill-rate and render-texture heavy, so the observed jump to 120 FPS after battery death is a strong signal that the flashlight/darkness pass is the bottleneck.

Files to inspect:

• src/systems/DarknessSystem.ts
• src/systems/FlashlightSystem.ts
• data/performance_budget.json

Acceptance target:

• Instrument real FPS in dev/public QA builds.
• Keep flashlight-on gameplay near the desktop target instead of ~30 FPS on a 1080p PC.
• The FPS delta between flashlight active and battery-dead states should be small enough that the flashlight state is not the dominant performance cliff.