Arrow icons, cascading own CSS, a preload for the hero image, and EB Solar

Built while setting up the redesign of ebsolar.de as the sixth local
installation (scripts/seed-ebsolar.php, reset-ebsolar.sh, ports 8096/8097).
The rebuild surfaced four things in the core:

- The icon set knows three arrows now: arrow-right, arrow-left,
  arrow-up-right. The button block and the post navigation render them as
  inline SVG inside the same aria-hidden span as before, so a template that
  hides the arrow with "& span{display:none}" keeps working. Until now the
  arrow was a text glyph that looked different in every font. Umut: "bei
  alle Projekte bzw bei allen möchte ich dieses emoji icon weg haben".

- Own CSS with the same nested selector twice kept only the last rule.
  "& .post-card{border:0}" followed by "& .post-card{transform:none}" lost
  the border - silently, which brought the core's card frame back on the
  project grid. The parser now joins the declarations in source order, as
  CSS does; the same for "&{…}" next to the element's own properties.

- The eager image over the fold was not preloaded. The background image of
  the first section has had a preload link for a while; a plain <img> was
  found only after the stylesheet was parsed, and that round trip went
  straight into LCP. PageController::heroImage() now writes the same src,
  srcset and sizes the block template writes, and page.html.twig emits the
  link - unless a background image is already preloaded.

- Measured on the way: an entrance effect on the LCP element costs about
  900 ms of render delay. The EB Solar hero image therefore has none.

Tests: 581 green, the icon test accepts quoted keys, two cases for the CSS
cascade, the preload asserted in the big-picture test.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/Content/DocumentValidator.php

@@ -16,7 +16,7 @@ final class DocumentValidator
     /** Blocks introduced with schema v6 for the sections a marketing page actually needs. */
     public const V6_TYPES = ['iconcard', 'stat', 'quote', 'logos', 'cta', 'contact', 'divider', 'spacer', 'accordion', 'accordionitem'];
     /** Icons ship as inline SVG from this fixed list; no icon font and no external request. */
-    public const ICONS = ['app', 'web', 'design', 'network', 'hardware', 'shop', 'support', 'shield', 'clock', 'star', 'phone', 'mail', 'pin', 'check', 'people', 'music', 'home'];
+    public const ICONS = ['app', 'web', 'design', 'network', 'hardware', 'shop', 'support', 'shield', 'clock', 'star', 'phone', 'mail', 'pin', 'check', 'people', 'music', 'home', 'arrow-right', 'arrow-left', 'arrow-up-right'];
     public const SPACER_SIZES = ['small', 'medium', 'large', 'huge'];
     /** Schema v7: tabs, the one pattern that needs a script to become a real widget. */
     public const V7_TYPES = ['tabs', 'tabitem'];