Add ContentHasher

Sources/Photochk/Core/ContentHasher.swift

@@ -0,0 +1,18 @@
+import CryptoKit
+import Foundation
+
+enum ContentHasher {
+    private static let chunkSize = 1 << 20  // 1 MB
+
+    /// ファイル全体を読み込まずに SHA-256 を求める。
+    static func sha256(of url: URL) throws -> String {
+        let handle = try FileHandle(forReadingFrom: url)
+        defer { try? handle.close() }
+
+        var hasher = SHA256()
+        while let chunk = try handle.read(upToCount: chunkSize), !chunk.isEmpty {
+            hasher.update(data: chunk)
+        }
+        return hasher.finalize().map { String(format: "%02x", $0) }.joined()
+    }
+}