Auto-install firmware from the game disc on boot

Most UMDs carry a firmware updater, and having a real firmware in the NAND is
what the LLE modules want. On boot, if the disc's updater is newer than what's
installed - or nothing identifiable is installed, which is what a fonts-only
NAND looks like - unpack it, with a progress bar on the OSD. Controlled by
bAutoUpgradeFirmware, on by default, with a checkbox on the firmware screen.
Off in headless, which shouldn't rewrite the NAND during a test run.

The install itself is now shared with the install screen, and hardened, since
it can run without anyone watching:

- It stages into <NAND>/install-staging and only erases the installed firmware
  once the new one is complete on disk, so a failure leaves what's there alone.
- A single entry that didn't unpack fails the whole install. A firmware with
  holes still looks installed, so nothing would ever replace it.
- A truncated archive is rejected instead of unpacking to half a firmware that
  every stat reports as a clean install. The PSAR header records the length;
  it was being clamped to the buffer rather than checked against it.

Also falls back to the version in the archive's own header when the PARAM.SFO
next to the updater is unreadable, which it is on a fair number of discs.

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

Core/System.cpp

@@ -63,6 +63,7 @@
 #include "Core/Config.h"
 #include "Core/Core.h"
 #include "Core/Util/PathUtil.h"
+#include "Core/Util/PSARUnpack.h"
 #include "Core/CoreTiming.h"
 #include "Core/CoreParameter.h"
 #include "Core/FileLoaders/RamCachingFileLoader.h"
@@ -513,6 +514,11 @@ static bool CPU_Init(FileLoader *fileLoader, IdentifiedFileType type, std::strin
 		g_CoreParameter.mountIsoLoader = ConstructFileLoader(g_CoreParameter.mountIso);
 	}
 
+	// Most game discs carry a firmware updater, so this is where a NAND with nothing (or only the
+	// fonts) in it gets filled in. Has to happen before the mount below: the install erases and
+	// rewrites the very directory flash0:/flash1: point at.
+	AutoInstallFirmwareFromDisc();
+
 	MountFileSystems();
 
 	// Game-specific settings are load from for example Load_PSP_ISO (which calls g_Config.LoadGameConfig).