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/block.html.twig

@@ -26,7 +26,7 @@
     {% endif %}
 {% elseif type == 'button' %}
     {% for device, settings in layout.button|default({}) %}{% for key, value in settings %}{% set designClasses = designClasses ~ ' b-' ~ device ~ '-' ~ key ~ '-' ~ value %}{% endfor %}{% endfor %}
-    <a id="{{ block.id }}" class="button cms-button{{ designClasses }}" href="{{ props.href }}"{% include 'blocks/link-attributes.html.twig' with {newTab: props.newTab|default(false)} only %}>{{ props.text }} <span aria-hidden="true">↗</span></a>
+    <a id="{{ block.id }}" class="button cms-button{{ designClasses }}" href="{{ props.href }}"{% include 'blocks/link-attributes.html.twig' with {newTab: props.newTab|default(false)} only %}>{{ props.text }} <span class="button-arrow" aria-hidden="true">{% include 'blocks/icon.html.twig' with {name: 'arrow-up-right'} only %}</span></a>
 {% elseif type == 'image' %}
     {# Wie breit das Bild tatsaechlich erscheint. Steht im Block eine feste Breite - ein Logo mit
        190px, ein Symbol mit 72px -, dann ist das die Antwort, und der Browser laedt die kleine