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

src/styles/plan.styles.ts

@@ -86,12 +86,21 @@ export const planStyles = css`
         opacity 1100ms cubic-bezier(.22, .61, .36, 1);
     }
     .stage.mode-transition .hp-day-cycle-env { transition: none; }
+    /* #532: the outline keeps its OWN compositing layer. The group holds paper
+       silhouettes only and does not change on hover or pan, but the filter used
+       to live in the plan's own layer — so every repaint of the plan re-ran
+       three blur passes over the whole sheet. Measured on a demo-stand pan
+       (Chromium, CDP, summed RasterTask): 852 ms without the hint against 28 ms
+       with it, where a static background costs 108 ms. The hint costs one layer
+       of memory, so it is scoped to .daycycle and a static background never
+       pays for it. */
     .stage.daycycle .hp-paperg,
     .hp-static-stage.daycycle .hp-paperg {
       filter:
         drop-shadow(0 0 1px var(--hp-day-cycle-outline-near))
         drop-shadow(0 0 5px var(--hp-day-cycle-outline-mid))
         drop-shadow(0 0 10px var(--hp-day-cycle-outline-far));
+      will-change: filter;
       transition: filter 1100ms cubic-bezier(.22, .61, .36, 1);
     }
     @media (prefers-reduced-motion: reduce) {