feat: pack readme names spellings left out

A pack folded for Windows and macOS keeps one spelling of each case twin. Extracted on a Steam Deck or Linux, the core asking for the other spelling finds nothing, and only the installer added it. Where the platform also runs on a filesystem that keeps case, the README now lists each spelling left out beside the one it holds: 12 for EmuDeck and 10 for RetroArch, the manifest's case_variants.

scripts/packreadme.py

@@ -207,6 +207,7 @@ def _build_readme(
     save_folder: str = "",
     save_where: str = "",
     save_by_system: bool = False,
+    case_twins: list[tuple[str, str]] | None = None,
 ) -> str:
     """Build a personalized step-by-step README for each platform pack.
 
@@ -216,6 +217,10 @@ def _build_readme(
     frontend's save directory, empty when the pack holds none, and
     ``save_where`` that directory's default place (save_place).
     ``save_by_system`` is set when the launcher hands each system a folder.
+    ``case_twins`` pairs each spelling the pack leaves out, a case twin of a
+    packed file, with the one it holds: given for a platform that also runs
+    where names keep their case (EmuDeck and RetroArch on Linux), where the
+    core asking for the other spelling finds nothing.
     """
     narrowings = narrowings or []
     sep = "=" * 50
@@ -245,6 +250,20 @@ def _build_readme(
             + (pack_manual(platform_name) or _generic_manual(bios_paths))
         )
 
+    if case_twins:
+        listed = "".join(
+            f"    {missing}  (the pack has {held})\n" for missing, held in case_twins
+        )
+        guide += (
+            "NAMES THAT DIFFER ONLY BY CASE\n\n"
+            "  Some cores open the same file under another spelling. This\n"
+            "  pack holds one spelling of each, all Windows and macOS can\n"
+            "  store in one folder. Linux, SteamOS and Android keep both:\n"
+            "  there, run the installer, which adds these spellings, or\n"
+            "  copy each file under the name below.\n\n"
+            f"{listed}\n"
+        )
+
     if save_folder:
         guide += _save_section(
             platform_display, save_folder, save_where, save_by_system,