feat(installer): add ChecksumVerifier (SHA256 + checksums.txt parser)

src/ClaudeDo.Installer/Core/ChecksumVerifier.cs

@@ -0,0 +1,37 @@
+using System.IO;
+using System.Security.Cryptography;
+
+namespace ClaudeDo.Installer.Core;
+
+public static class ChecksumVerifier
+{
+    public static string ComputeSha256(string filePath)
+    {
+        using var stream = File.OpenRead(filePath);
+        using var sha = SHA256.Create();
+        var hash = sha.ComputeHash(stream);
+        return Convert.ToHexString(hash).ToLowerInvariant();
+    }
+
+    public static bool Verify(string filePath, string expectedSha256)
+    {
+        var actual = ComputeSha256(filePath);
+        return string.Equals(actual, expectedSha256.Trim(), StringComparison.OrdinalIgnoreCase);
+    }
+
+    public static IReadOnlyDictionary<string, string> ParseChecksumsFile(string content)
+    {
+        var map = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        foreach (var rawLine in content.Split('\n'))
+        {
+            var line = rawLine.Trim();
+            if (line.Length == 0) continue;
+            var parts = line.Split(new[] { ' ', '\t' }, 2, StringSplitOptions.RemoveEmptyEntries);
+            if (parts.Length != 2) continue;
+            var hashPart = parts[0].Trim();
+            if (hashPart.Length != 64) continue;
+            map[parts[1].Trim()] = hashPart;
+        }
+        return map;
+    }
+}