The battery pass: paint less, wake less

- opaque canvas: the compositor stops alpha-blending 2.5Mpx at 60Hz
- settled modals and the death summary idle at 20-24 fps; play untouched
- TUNE.lowPower caps RES at 3 (-44% pixels/frame), flippable live

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

config.js

@@ -312,6 +312,9 @@ export const TUNE = {
   barrelHp: 1,      // anything pops it: ball, pellet, bomb, flame, swish. It is a target, not a body
   barrelH: 1.15,    // drawn height (a body is 1.8)
   fps: 60,          // frame cap — 30 halves battery/heat but Oliver felt it; smoothness wins (0 = uncapped)
+  lowPower: 0,      // >= 0.5 caps RES at 3 (−44% of pixels painted per frame) — the battery-saver
+                    // switch, flippable live from the T panel. Play speed and fps are untouched:
+                    // the trade is sharpness only, and 224px sprites keep it "softer", never mushy.
   runnerSpeed: 3.2, // runner ('f') speed multiplier over the shamble
   runnerVMax: 4.2,  // hard runner speed cap — the late-game ramp must stay humanly reactable
   shamblerHp: 2,    // splat-quality hits a classic shambler ('a'/'b') soaks before it dies — only the runner ('f') is still one-shot glass