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>

dom.js

@@ -1,3 +1,6 @@
 // Canvas handles — imported by everyone, imports nothing (keeps module cycles harmless).
 export const cv = document.getElementById('c');
-export const ctx = cv.getContext('2d');
+// alpha:false — render() clears the whole frame to an opaque colour anyway, so declaring the canvas
+// opaque lets the compositor skip blending a 2.5-megapixel layer 60 times a second (battery, not
+// looks). desynchronized is a further no-op-where-unsupported compositor shortcut (Chrome).
+export const ctx = cv.getContext('2d', { alpha: false, desynchronized: true });