Add concurrency helper

Sources/Photochk/Core/Concurrency.swift

@@ -0,0 +1,42 @@
+import Foundation
+
+/// 同時実行数を抑えつつ並列に変換し、入力と同じ順序で結果を返す。
+///
+/// 画像デコードや Vision の推論は CPU を使い切るので、無制限に投げるより
+/// コア数程度に抑えたほうが速く、メモリも安定する。
+func mapConcurrently<Item: Sendable, Output: Sendable>(
+    _ items: [Item],
+    limit: Int = max(2, ProcessInfo.processInfo.activeProcessorCount),
+    onProgress: (@Sendable (Int) -> Void)? = nil,
+    _ transform: @escaping @Sendable (Item) async -> Output
+) async throws -> [Output] {
+    guard !items.isEmpty else { return [] }
+
+    var results = [Output?](repeating: nil, count: items.count)
+
+    try await withThrowingTaskGroup(of: (Int, Output).self) { group in
+        var next = 0
+        let initial = min(limit, items.count)
+        while next < initial {
+            let index = next
+            group.addTask { (index, await transform(items[index])) }
+            next += 1
+        }
+
+        var finished = 0
+        while let (index, value) = try await group.next() {
+            results[index] = value
+            finished += 1
+            onProgress?(finished)
+            try Task.checkCancellation()
+
+            if next < items.count {
+                let index = next
+                group.addTask { (index, await transform(items[index])) }
+                next += 1
+            }
+        }
+    }
+
+    return results.compactMap { $0 }
+}