feat(runtime): add CHD discs and mod-owned loading modes

runtime/src/disc_path.cpp

@@ -18,6 +18,7 @@ namespace {
 const char* const kImageExtensions[] = { ".bin", ".img", ".iso" };
 
 bool is_cue(const fs::path& p) { return path_has_extension_ci(p, ".cue"); }
+bool is_chd(const fs::path& p) { return path_has_extension_ci(p, ".chd"); }
 
 bool is_raw_image(const fs::path& p) {
     for (const char* ext : kImageExtensions) {
@@ -121,6 +122,10 @@ DiscPathResolution resolve_disc_path(const fs::path& picked) {
 
     if (r.picked.empty()) return r;
 
+    // CHD is already a complete image + table of contents. It must not be
+    // upgraded to a same-stem cue or treated as that cue's raw payload.
+    if (is_chd(r.picked)) return r;
+
     if (is_cue(r.picked)) {
         const CueSheet sheet = parse_cue_sheet(r.picked);
         if (sheet.usable()) {