4.1 ImageStore

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/FrameProcessor/Storage/ImageStore.cs

@@ -0,0 +1,52 @@
+using System.Diagnostics.CodeAnalysis;
+using FrameProcessor.Configuration;
+using FrameProcessor.Domain;
+using Microsoft.Extensions.Options;
+
+namespace FrameProcessor.Storage;
+
+/// <summary>
+/// Persists the latest processed PNG for each frame to <see cref="StorageOptions.ImageDirectory"/>.
+/// Writes are atomic via a temp file + <see cref="File.Move(string, string, bool)"/> rename so the
+/// fetch endpoint never observes a partial file (see SPEC.md §7.2, CLAUDE.md "Atomic writes only").
+/// </summary>
+public sealed class ImageStore
+{
+    private readonly string _directory;
+
+    public ImageStore(IOptions<StorageOptions> options)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        _directory = options.Value.ImageDirectory;
+        Directory.CreateDirectory(_directory);
+    }
+
+    public async Task WriteAsync(MacAddress mac, ReadOnlyMemory<byte> bytes, CancellationToken cancellationToken)
+    {
+        var finalPath = GetPath(mac);
+        var tmpPath = finalPath + ".tmp";
+
+        await using (var stream = new FileStream(tmpPath, FileMode.Create, FileAccess.Write, FileShare.None))
+        {
+            await stream.WriteAsync(bytes, cancellationToken).ConfigureAwait(false);
+            await stream.FlushAsync(cancellationToken).ConfigureAwait(false);
+        }
+
+        File.Move(tmpPath, finalPath, overwrite: true);
+    }
+
+    public bool TryGetPath(MacAddress mac, [NotNullWhen(true)] out string? path)
+    {
+        var candidate = GetPath(mac);
+        if (File.Exists(candidate))
+        {
+            path = candidate;
+            return true;
+        }
+
+        path = null;
+        return false;
+    }
+
+    private string GetPath(MacAddress mac) => Path.Combine(_directory, $"{mac}.png");
+}