feat: permanent download page per platform

get/<platform>/ lists the files of the current release pack, read from
release.json, or gives the command for a setup without one. The README,
setup pages, Get your files and the release notes link it instead of
asset URLs, which die with each deleted tag. Setups without a ZIP say so
with the command beside it, and validate_site checks every get/ page.

scripts/download.sh

@@ -115,6 +115,8 @@ EOF
         echo "Error: Platform '$platform' not found in latest release."
         echo "Available platforms:"
         platform_names "$release_json"
+        echo "A setup without a pack is served by the installer:"
+        echo "  curl -fsSL https://raw.githubusercontent.com/Abdess/retrobios/main/install.sh | sh -s -- --platform <name>"
         exit 1
     fi