fix(ds): guard swapScreen KVC access with responds(to:) to prevent crash

The dual-screen viewport code accessed `swapScreen` via KVC without
checking whether the bridge implements it, causing an
NSUndefinedKeyException for melonDS, DeSmuME2015 and ThinLibretro DS
bridges.

- Use responds(to:) before value(forKey: "swapScreen") in isScreenSwapped
- Replace hardcoded "com.provenance.ds" with SystemIdentifier.DS.rawValue

Fixes Copilot review issues on PR #3377.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: github-actions[bot]

PVCoreBridgeRetro/Sources/PVLibRetro/PVThinLibretroCore.swift

@@ -89,7 +89,7 @@ class PVThinLibretroCore: PVEmulatorCore {
     /// DS (NDS) cores output a combined 256×384 framebuffer (top screen + bottom screen).
     /// The DefaultDeltaSkin NDS layout splits this into two independently positioned viewports.
     public override var supportsDualScreens: Bool {
-        systemIdentifier == "com.provenance.ds"
+        systemIdentifier == SystemIdentifier.DS.rawValue
     }
 
     required init() {

PVUI/Sources/PVUIBase/PVEmulatorVC/PVEmulatorViewController+DualScreen.swift

@@ -24,9 +24,11 @@ extension PVEmulatorViewController {
 
     /// Get swap screen state from emuThreeDS core
     private var isScreenSwapped: Bool {
-        guard let bridge = emuThreeBridge else { return false }
-        // Access swapScreen property via runtime
-        if let value = (bridge as AnyObject).value(forKey: "swapScreen") as? Bool {
+        guard let bridge = emuThreeBridge as? NSObject else { return false }
+        // Guard against NSUndefinedKeyException for bridges that don't expose swapScreen
+        let sel = NSSelectorFromString("swapScreen")
+        guard bridge.responds(to: sel) else { return false }
+        if let value = bridge.value(forKey: "swapScreen") as? Bool {
             return value
         }
         return false