fix(file-provider): static DateFormatter + edge-case tests

- Extract DateFormatter in FileProviderItem.filename (saveStateFile) to a
  private static lazy property — eliminates repeated allocation on every
  filename access without changing output behaviour
- Add edge-case tests: parseSaveStateID/parseSaveStateGameMD5 with empty
  content after prefix, parseScreenshotID must not match sc-game: prefix,
  screenshotItemIdentifier round-trip with zero index

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

Author: github-actions[bot]

Extensions/ROM File Provider/FileProviderItem.swift

@@ -251,11 +251,7 @@ final class FileProviderItem: NSObject, NSFileProviderItem {
         case .saveStateFile(let id, _, let date, let isAutosave, let userDescription, let fileURL, _):
             if let name = fileURL?.lastPathComponent, !name.isEmpty { return sanitize(name) }
             let label = isAutosave ? "Auto" : (userDescription ?? "")
-            let formatter = DateFormatter()
-            formatter.locale = Locale(identifier: "en_US_POSIX")
-            formatter.timeZone = TimeZone(secondsFromGMT: 0)
-            formatter.dateFormat = "yyyy-MM-dd HH-mm-ss"
-            let dateStr = formatter.string(from: date)
+            let dateStr = FileProviderItem.saveStateDateFormatter.string(from: date)
             let prefix = label.isEmpty ? dateStr : "\(label) \(dateStr)"
             return sanitize("\(prefix) \(id.prefix(8)).pvs")
         case .screenshotGameFolder(let game, _):
@@ -425,6 +421,15 @@ final class FileProviderItem: NSObject, NSFileProviderItem {
         return attributes?[.modificationDate] as? Date
     }()
 
+    /// Shared formatter for save state filenames — created once (DateFormatter is expensive).
+    private static let saveStateDateFormatter: DateFormatter = {
+        let f = DateFormatter()
+        f.locale = Locale(identifier: "en_US_POSIX")
+        f.timeZone = TimeZone(secondsFromGMT: 0)
+        f.dateFormat = "yyyy-MM-dd HH-mm-ss"
+        return f
+    }()
+
     /// Sanitizes a raw name for safe use as a file provider filename.
     private func sanitize(_ raw: String) -> String {
         let withoutSeparators = raw

PVPrimitives/Tests/PVPrimitivesTests/RomFileProviderVirtualPathTests.swift

@@ -81,6 +81,10 @@ struct RomFileProviderVirtualPathTests {
     @Test func parseSaveStateID_invalidPrefix() {
         #expect(RomFileProviderVirtualPath.parseSaveStateID(from: "game:ABCD") == nil)
         #expect(RomFileProviderVirtualPath.parseSaveStateGameMD5(from: "rating:5") == nil)
+        // Empty id after prefix
+        #expect(RomFileProviderVirtualPath.parseSaveStateID(from: "ss:") == nil)
+        // Empty MD5 after folder prefix
+        #expect(RomFileProviderVirtualPath.parseSaveStateGameMD5(from: "ss-game:") == nil)
     }
 
     // MARK: - Screenshot identifiers
@@ -105,6 +109,19 @@ struct RomFileProviderVirtualPathTests {
     @Test func parseScreenshotID_invalidPrefix() {
         #expect(RomFileProviderVirtualPath.parseScreenshotID(from: "game:ABCD") == nil)
         #expect(RomFileProviderVirtualPath.parseScreenshotGameMD5(from: "year:1998") == nil)
+        // Empty MD5 after folder prefix
+        #expect(RomFileProviderVirtualPath.parseScreenshotGameMD5(from: "sc-game:") == nil)
+        // sc-game: prefix must NOT be parsed as a screenshot item (sc: prefix)
+        let scGameRaw = "sc-game:" + String(repeating: "A", count: 32)
+        #expect(RomFileProviderVirtualPath.parseScreenshotID(from: scGameRaw) == nil)
+    }
+
+    @Test func screenshotItemIdentifier_zeroIndex() {
+        let md5 = String(repeating: "D", count: 32)
+        let id = RomFileProviderVirtualPath.screenshotItemIdentifier(gameMD5: md5, index: 0)
+        let parsed = RomFileProviderVirtualPath.parseScreenshotID(from: id)
+        #expect(parsed?.gameMD5 == md5.uppercased())
+        #expect(parsed?.index == 0)
     }
 
     // MARK: - Root categories