Keep the day-cycle outline on its own compositing layer

The "Follows the sun" background made the plan crawl in Firefox: 23 MB of
textures per frame, sixteen of about twenty picture-cache tiles thrown
away every frame, nine frames per second. The card's own JavaScript was
idle for 89 % of that.

The cost is the outer outline. It is a triple drop-shadow over the
grouped paper footprint, and although the group holds paper silhouettes
only and never changes on hover or pan, the filter lived in the plan's
own layer — so every repaint of the plan re-ran three blur passes over
the whole sheet. One hint moves the filtered paper into its own layer
and unhooks it from the plan's repaints.

Measured on a demo-stand pan (Chromium, CDP, summed RasterTask): 1456.6
ms against a static background's 96.7 ms before, 84.7 ms against 103.2
ms after. The new smoke measures that ratio and fails above two.

The picture does not change: the outline outside the plan matches byte
for byte and the frame's mean colour moves from 176.59 to 176.66 of 255.
Splitting the layer does move the remaining content onto a different
sub-pixel grid, so the diagonal wall hatch anti-aliases differently and
the four day-cycle baselines are re-taken on the beta candidate.

Issue: #532
User-Visible: yes

scripts/mutation-gate.mjs

@@ -2400,6 +2400,18 @@ const MUTANT_DEFINITIONS = [
       replace: "const HANDLE_PAINT_ORDER = ['corners', 'edges'] as const;",
     }],
   },
+  {
+    id: 'daycycle-outline-not-promoted',
+    guard: 'node demo/smoke_daycycle_raster.mjs',
+    because: 'the day-cycle outline is a triple drop-shadow over the whole sheet; without its own '
+      + 'compositing layer every repaint of the plan re-runs three blur passes, and a demo-stand '
+      + 'pan costs fifteen times the rasterization of a static background (#532 AC1/AC2)',
+    patches: [{
+      file: 'src/styles/plan.styles.ts',
+      find: '      will-change: filter;\n',
+      replace: '',
+    }],
+  },
   {
     id: 'align-guides-exclude-dead-source',
     guard: 'node demo/smoke_align_guides.mjs',