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src/Policy/StellaOps.Policy.Engine/Overlay/FileOverlayStore.cs

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+using System.Text;
+using System.Text.Json;
+using Microsoft.Extensions.Hosting;
+
+namespace StellaOps.Policy.Engine.Overlay;
+
+/// <summary>
+/// Persists overlay projections as NDJSON files under overlay/{tenant}/{ruleId}/{version}.ndjson.
+/// </summary>
+internal sealed class FileOverlayStore : IOverlayStore
+{
+    private readonly string _root;
+
+    public FileOverlayStore(IHostEnvironment env)
+    {
+        _root = Path.Combine(env.ContentRootPath, "overlay");
+    }
+
+    public async Task SaveAsync(OverlayProjection projection, CancellationToken cancellationToken = default)
+    {
+        if (projection is null)
+        {
+            throw new ArgumentNullException(nameof(projection));
+        }
+
+        var tenant = Sanitize(projection.Tenant);
+        var rule = Sanitize(projection.RuleId);
+        var dir = Path.Combine(_root, tenant, rule);
+        Directory.CreateDirectory(dir);
+
+        var path = Path.Combine(dir, $"{projection.Version}.ndjson");
+        var lines = new[]
+        {
+            JsonSerializer.Serialize(new OverlayProjectionHeader("overlay-projection-v1")),
+            JsonSerializer.Serialize(projection)
+        };
+        await File.WriteAllLinesAsync(path, lines, Encoding.UTF8, cancellationToken).ConfigureAwait(false);
+    }
+
+    private static string Sanitize(string value)
+    {
+        foreach (var ch in Path.GetInvalidFileNameChars())
+        {
+            value = value.Replace(ch, '_');
+        }
+
+        return string.IsNullOrWhiteSpace(value) ? "unknown" : value;
+    }
+}