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>

templates/blocks/icon.html.twig

@@ -16,6 +16,9 @@
     check: 'm4 12.5 5.5 5.5L20 6',
     people: 'M9.5 11.5a3.5 3.5 0 1 0 0-7 3.5 3.5 0 0 0 0 7ZM2.5 20v-1c0-3.3 3-5 7-5s7 1.7 7 5v1M16.5 5.3a3.2 3.2 0 0 1 0 6.4M18 14.4c2.1.7 3.5 2.1 3.5 4.1V20',
     music: 'M10 18V5.5L20 3v12.5M10 9.5 20 7M10 18a3 2.5 0 1 0-6 0 3 2.5 0 0 0 6 0ZM20 15.5a3 2.5 0 1 0-6 0 3 2.5 0 0 0 6 0Z',
-    home: 'm3 11.5 9-7.5 9 7.5M5.5 10v10h13V10M10 20v-6.5h4V20'
+    home: 'm3 11.5 9-7.5 9 7.5M5.5 10v10h13V10M10 20v-6.5h4V20',
+    'arrow-right': 'M4 12h15.5m-6.5-6.5 6.5 6.5-6.5 6.5',
+    'arrow-left': 'M20 12H4.5M11 5.5 4.5 12 11 18.5',
+    'arrow-up-right': 'M6.5 17.5 17.5 6.5M8.5 6.5h9v9'
 } %}
 <svg class="block-icon" viewBox="0 0 24 24" width="32" height="32" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true" focusable="false"><path d="{{ paths[name] }}"/></svg>