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>

public/assets/site.css

@@ -44,6 +44,11 @@ p{hyphens:auto}
 
 /* Blocks introduced with schema v6. No JavaScript: the accordion is details/summary. */
 .block-icon{flex-shrink:0;display:block}
+/* Der Pfeil am Button und in der Beitragsnavigation ist dasselbe Symbol wie ueberall sonst -
+   frueher stand dort ein Schriftzeichen, das je nach Schrift anders aussah. Er waechst mit der
+   Schrift des Knopfs, statt eine eigene Groesse zu haben. */
+.button-arrow,.nav-arrow{display:inline-flex;align-items:center}
+.button-arrow>svg,.nav-arrow>svg{width:1.15em;height:1.15em}
 .block-iconcard{display:grid;gap:12px;padding:26px;border:1px solid currentColor;border-radius:12px}
 .block-iconcard + .block-iconcard,.block-quote + .block-quote,.block-cta + .block-cta,.block-iconcard + .block-quote,.block-quote + .block-iconcard{margin-top:18px}
 /* Gestapelt brauchen Karten den Abstand; nebeneinander in einer Zeile macht er die zweite bis