headless: make --nand and --screenshot-save work

--nand had a field and an ApplyToConfig branch but nothing ever parsed it, so it
was silently ignored.

--screenshot-save only fired when a GE replay finished or a test used the
EMIT_SCREENSHOT devctl, and even then only under --compare. Anything else - a
game, or --vsh - ran to the timeout and wrote nothing. Capture the display at
the end of the run when nothing else did, which is what makes it usable for
looking at what a booting system actually has on screen.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Core/CmdLine.cpp

@@ -194,6 +194,7 @@ static const CommandLineParam g_autoParams[] = {
 	{POFF(appendConfig), CmdParamType::String, "appendconfig", '\0', "Merge config FILE into the current configuration"},
 	{POFF(root), CmdParamType::String, "root", 'r', "Mount directory as the root of host0:/."},
 	{POFF(memStick), CmdParamType::String, "memstick", '\0', "Memory stick root directory (contains PSP/GAME etc)"},
+	{POFF(nand), CmdParamType::String, "nand", '\0', "Root NAND directory, one level above flash0 (default: the memstick's PSP/NAND)"},
 	{POFF(stateToLoad), CmdParamType::String, "state", '\0', "Load state from specified file"},
 	{POFF(stateToSave), CmdParamType::String, "save-state", '\0', "Save a state to FILE partway through the run", CmdLineMode::Headless},
 	{POFF(compare), CmdParamType::Bool, "compare", 'c', "Enable comparison mode", CmdLineMode::Headless},