VSH: An empty UMD drive, and inserting a disc over the WebSocket debugger

The XMB, booted without a game, now starts with an empty drive: when
vsh_module loads with nothing at disc0:, the drive is marked empty, and
activating it tells the shell's drive callback (SceVshMediaDetectUMD)
there's no disc. Activation used to report a readable disc regardless,
so the shell asked for its type and showed "The disc could not be read."
The disc type devctl (0x01F20001) answers 0 without a disc, as JPCSP
does.

game.umd.insert (path) mounts a disc image into the empty drive and
calls the drive callback as inserting one does; game.umd.eject takes it
out. UmdMount() is UmdReplace() without its need for a disc already in.

Inserted into the 6.61 XMB, Daxter shows in the Game column with its
icon, background and animated preview, and launching it boots the game
to its title menu.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/Loaders.cpp

@@ -353,7 +353,10 @@ bool UmdReplace(const Path &filepath, FileLoader **fileLoader, std::string &erro
 		error = "has no disc";
 		return false;
 	}
+	return UmdMount(filepath, fileLoader, error);
+}
 
+bool UmdMount(const Path &filepath, FileLoader **fileLoader, std::string &error) {
 	FileLoader *loadedFile = ConstructFileLoader(filepath);
 
 	if (!loadedFile || !loadedFile->Exists()) {
@@ -374,7 +377,7 @@ bool UmdReplace(const Path &filepath, FileLoader **fileLoader, std::string &erro
 	case IdentifiedFileType::PSP_ISO_NP:
 	case IdentifiedFileType::PSP_DISC_DIRECTORY:
 		if (!MountGameISO(loadedFile, &error)) {
-			error = "mounting the replaced ISO failed: " + error;
+			error = "mounting the ISO failed: " + error;
 			return false;
 		}
 		break;