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>

render.js

@@ -58,7 +58,10 @@ const IS_TOUCH = matchMedia('(pointer: coarse)').matches; // hoisted: a matchMed
 // nearly double the pixels it can show, on exactly the hardware least able to afford it. The same
 // near-1:1 logic per tier: dpr 3 -> 4, dpr 2 -> 3, dpr 1 -> 2.
 // THIS IS THE FIRST KNOB TO TURN DOWN if the phone ever stutters: it is the whole fill-rate budget.
-export const RES = Math.max(2, Math.min(4, Math.ceil((devicePixelRatio || 3) * 1.35)));
+// `let`, not const: TUNE.lowPower caps it at 3 live (fitViewport re-reads on every fit) — importers
+// see the change through the module binding.
+const RES_AUTO = Math.max(2, Math.min(4, Math.ceil((devicePixelRatio || 3) * 1.35)));
+export let RES = RES_AUTO;
 export let W = 480, H = 270;
 
 // Split responsibility, and the split is deliberate:
@@ -111,6 +114,10 @@ function fitBox() {
 
 export function fitViewport() { // landscape 480x270; portrait 270 wide and as TALL as the phone is
   fitBox();
+  // battery saver: RES 4 -> 3 is −44% of pixels painted per frame, by far the biggest lever there
+  // is. Sprites are 224px sources, so at 3 they still draw at-or-below 1:1 — softer than 4, never
+  // mushy. main.js re-calls fitViewport when the panel flips the knob.
+  RES = TUNE.lowPower >= 0.5 ? Math.min(3, RES_AUTO) : RES_AUTO;
   const portrait = (cv.clientHeight || innerHeight) > (cv.clientWidth || innerWidth);
   // Portrait height ADAPTS to the device. A fixed 270x480 is 9:16, but a modern iPhone is taller
   // (~390x844 = 0.46), so width bound the fit and ~150 CSS px of screen went unused at the bottom.