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/page.html.twig

@@ -34,6 +34,10 @@
        drei Abfragen trifft zu - und zwar auf dieselbe Variante, die auch die Regel nimmt. #}
     {% for bild in heroBackground|default([]) %}<link rel="preload" as="image" href="{{ bild.src }}" media="{{ bild.media }}" fetchpriority="high">
     {% endfor %}
+    {# Und derselbe Dienst fuer das gewoehnliche Bild ueber der Falz: ohne diese Zeile faengt der
+       Browser es erst an zu holen, wenn er das Stylesheet geparst hat. #}
+    {% if heroImage|default(null) %}<link rel="preload" as="image" href="{{ heroImage.src }}"{% if heroImage.srcset %} imagesrcset="{{ heroImage.srcset }}" imagesizes="{{ heroImage.sizes }}"{% endif %} fetchpriority="high">
+    {% endif %}
     {% if localFonts|default([]) %}
     {# Jeder Schnitt, den das Theme nennt, vorab - und zwar alle. Fehlt einer, laedt er erst,
        wenn das Stylesheet geparst ist, und die Ueberschrift springt sichtbar von der