feat: record files a launcher never opens

unread_on names the platforms whose launcher points the emulator at other files. RetroBat writes absolute BIOS paths into standalone BlastEm's config, so the default names were reported as required and missing there. Such an entry leaves that platform's gap report and pack, and its emulator page says where it is never opened.

scripts/generate_site.py

@@ -2861,6 +2861,9 @@ def _render_emulator_file(
     if f.get("not_bundled_on"):
         details.append("Left out by the platform's package, so the pack brings it, on: "
                        + ", ".join(sorted(f["not_bundled_on"])))
+    if f.get("unread_on"):
+        details.append("Never opened on these platforms, whose launcher names other files: "
+                       + ", ".join(sorted(f["unread_on"])))
     if source_ref:
         asker = requested_by(profile, source_ref)
         if asker: