Add a colour wash animation

The frame parks in the middle and the colour changes instead: a gradient
creeps round the colour wheel behind the poster, and the frame's glow
travels with it. Every other animation moves the frame, so the still one
reads as a rest between them rather than as a dead spot.

The wash layer blends with mix-blend-mode: color, which keeps the
poster's luminance and replaces only its hue. soft-light was the first
try and barely shifted the colour at all against a poster already knocked
back to 45% brightness.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tg15tLAuT6NrCE1ZxKVgDB

public/player/motion.js

@@ -19,6 +19,9 @@ import {
   TRAIL_SPACING,
   TRAIL_LENGTH,
   TRAIL_FADE_SECONDS,
+  WASH_CYCLE_SECONDS,
+  WASH_SPREAD,
+  WASH_STRENGTH,
   AUTO_ANIMATION,
 } from '/config.js';
 
@@ -903,6 +906,78 @@ export function startDragResize({ element }) {
   };
 }
 
+// --- colour wash --------------------------------------------------------
+
+// The frame parks in the middle and the room changes colour around it: a wash
+// creeps round the colour wheel behind the poster, and the frame's glow travels
+// with it, so the whole screen shifts as one. It is the quiet one — everything
+// else in the registry moves the frame, and after a few of those a still frame
+// with a slow colour behind it is a relief rather than a dead spot.
+
+/**
+ * The two ends of the wash gradient at `elapsed` seconds, as hues in degrees.
+ * One trip round the wheel takes WASH_CYCLE_SECONDS, and the second hue trails
+ * the first by WASH_SPREAD so the gradient always has somewhere to go. Pure, so
+ * the cycle can be exercised without a browser.
+ */
+export function washHues(elapsed) {
+  const turn = ((elapsed / WASH_CYCLE_SECONDS) * 360) % 360;
+  return [turn, (turn + WASH_SPREAD) % 360];
+}
+
+/** A wash stop as a colour: saturated and mid-bright, before the blend mode. */
+function washStop(hue, alpha = 1) {
+  return `hsl(${hue.toFixed(1)} 85% 55% / ${alpha})`;
+}
+
+// The frame's glow is the wash colour held back to the same weight the fixed
+// pink had, so a hue with more punch to it does not brighten the whole frame.
+const GLOW_ALPHA = 0.55;
+
+export function startWash({ element }) {
+  const wash = document.createElement('div');
+  wash.id = 'wash';
+  wash.style.opacity = String(WASH_STRENGTH);
+  // Behind the stills and the frame, in front of the poster it tints.
+  document.body.appendChild(wash);
+  element.classList.add('washing');
+
+  function centre() {
+    const size = applyFrameSize(element);
+    place(element, (window.innerWidth - size.width) / 2, (window.innerHeight - size.height) / 2);
+  }
+
+  centre();
+  window.addEventListener('resize', centre);
+
+  const started = performance.now();
+  let handle = 0;
+  let stopped = false;
+
+  function frame(now) {
+    if (stopped) return;
+
+    const [from, to] = washHues((now - started) / 1000);
+    wash.style.background = `linear-gradient(135deg, ${washStop(from)}, ${washStop(to)})`;
+    // The frame's glow rides the leading hue, so it belongs to the wash instead
+    // of sitting on top of it in a fixed pink.
+    element.style.setProperty('--wash', washStop(from, GLOW_ALPHA));
+
+    handle = requestAnimationFrame(frame);
+  }
+
+  handle = requestAnimationFrame(frame);
+
+  return function stop() {
+    stopped = true;
+    cancelAnimationFrame(handle);
+    window.removeEventListener('resize', centre);
+    wash.remove();
+    element.classList.remove('washing');
+    element.style.removeProperty('--wash');
+  };
+}
+
 // --- registry -----------------------------------------------------------
 
 // Keyed by the names in config.js, which is what travels over the socket.
@@ -911,6 +986,7 @@ export const ANIMATIONS = {
   corners: startCorners,
   drag: startDragResize,
   trail: startTrail,
+  wash: startWash,
 };
 
 const NAMES = Object.keys(ANIMATIONS);