sceReg: Let a key read a bool setting, use it for VSH UMD autoboot

KeyValue gets an optional pointer to a g_Config bool, which the key
reads as 0/1. The setting wins over guest writes: since category_version
is kept low, the VSH upgrades the registry at every boot and rewrites
every key with its default.

/CONFIG/SYSTEM/umd_autoboot reads the new SystemParam VshUmdAutoboot
(default true, as before). Turned off, --vsh --mount stays in the XMB
with the disc in the Game column instead of starting it.

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

Core/Config.cpp

@@ -1121,6 +1121,7 @@ static const ConfigSetting systemParamSettings[] = {
 	ConfigSetting("BypassOSKWithKeyboard", SETTING(g_Config, bBypassOSKWithKeyboard), false, CfgFlag::PER_GAME),
 	ConfigSetting("WlanPowerSave", SETTING(g_Config, bWlanPowerSave), (bool) PSP_SYSTEMPARAM_WLAN_POWERSAVE_OFF, CfgFlag::PER_GAME),
 	ConfigSetting("EncryptSave", SETTING(g_Config, bEncryptSave), true, CfgFlag::PER_GAME | CfgFlag::REPORT),
+	ConfigSetting("VshUmdAutoboot", SETTING(g_Config, bVshUmdAutoboot), true, CfgFlag::DEFAULT),
 	ConfigSetting("MemStickSize", SETTING(g_Config, iMemStickSizeGB), 16, CfgFlag::DEFAULT),
 	ConfigSetting("FuseID", SETTING(g_Config, sFuseID), "", CfgFlag::DEFAULT),
 };