JR-010: record how truth data was obtained

Three routes now deliver truth data -- a sidecar, a worker push, and a fetched
manifest -- and once stored they were indistinguishable. The truth file records
nothing about how it arrived, so a locally computed sidecar and a loose-tier
manifest from a third-party server looked identical to every reader, despite
making claims of very different strength about the same item.

TruthProvenance records source, server, match tier, applied offset and caveat.
GET /Items/{itemId}/Provenance serves it, and JR-036's loose-tier caveat now
has somewhere to come from.

Two decisions carried in the code rather than assumed:

Provenance is stored BESIDE the truth file, never inside it. Injecting fields
would mean the bytes served back are not the bytes the producer wrote, which is
the property JR-004 turns on. UT-026 pins it by asserting the stored truth JSON
contains no provenance keys.

The applied offset is recorded because it is otherwise unrecoverable. Once
JR-030 shifts every window the timings look native, and nothing else would say
they had been shifted -- which matters when diagnosing an overlay that is
consistently a few seconds out.

A sidecar's provenance is derived rather than stored: it is local, and its
timestamp is the file's own. Precedence resolves through the same rule as
GetTruthAsync, because resolving it twice by different rules is how the two
would drift.

Fourth mutation check: stopping Delete from removing provenance fails UT-027
alone -- a stale record would otherwise outlive its claim and describe data the
next fetch had already replaced.

TRACES: UT-024, UT-025, UT-026, UT-027, UT-028 | JR-010

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Jellyfin.Plugin.JRay/Services/ManagedTruthStore.cs

@@ -60,7 +60,7 @@ public sealed class ManagedTruthStore : IManagedTruthStore
     }
 
     /// <inheritdoc />
-    public async Task SaveAsync(Guid itemId, TruthFile truth, CancellationToken cancellationToken)
+    public async Task SaveAsync(Guid itemId, TruthFile truth, TruthProvenance provenance, CancellationToken cancellationToken)
     {
         var path = GetPath(itemId);
         var directory = Path.GetDirectoryName(path) ?? throw new InvalidOperationException("Managed truth path has no directory.");
@@ -73,11 +73,52 @@ public sealed class ManagedTruthStore : IManagedTruthStore
         }
 
         File.Move(tempPath, path, overwrite: true);
+
+        // Written after the truth file, so a crash between the two leaves truth
+        // with no provenance (readable, source unknown) rather than provenance
+        // describing a file that is not there.
+        var provenancePath = GetProvenancePath(itemId);
+        var provenanceTemp = provenancePath + ".tmp";
+        using (var stream = File.Create(provenanceTemp))
+        {
+            await JsonSerializer.SerializeAsync(stream, provenance, JsonOptions, cancellationToken).ConfigureAwait(false);
+        }
+
+        File.Move(provenanceTemp, provenancePath, overwrite: true);
+    }
+
+    /// <inheritdoc />
+    public TruthProvenance? LoadProvenance(Guid itemId)
+    {
+        var path = GetProvenancePath(itemId);
+        if (!File.Exists(path))
+        {
+            return null;
+        }
+
+        try
+        {
+            using var stream = File.OpenRead(path);
+            return JsonSerializer.Deserialize<TruthProvenance>(stream, JsonOptions);
+        }
+        catch (Exception ex) when (ex is IOException or JsonException)
+        {
+            // Provenance is metadata about the claim, not the claim. Losing it
+            // must never make readable truth data unreadable.
+            _logger.LogWarning(ex, "JRay: failed to read truth provenance at {Path}", path);
+            return null;
+        }
     }
 
     /// <inheritdoc />
     public bool Delete(Guid itemId)
     {
+        var provenancePath = GetProvenancePath(itemId);
+        if (File.Exists(provenancePath))
+        {
+            File.Delete(provenancePath);
+        }
+
         var path = GetPath(itemId);
         if (!File.Exists(path))
         {
@@ -98,4 +139,9 @@ public sealed class ManagedTruthStore : IManagedTruthStore
     {
         return Path.Combine(_applicationPaths.PluginConfigurationsPath, "JRay", "truth", itemId.ToString("D") + ".json");
     }
+
+    private string GetProvenancePath(Guid itemId)
+    {
+        return Path.Combine(_applicationPaths.PluginConfigurationsPath, "JRay", "truth", itemId.ToString("D") + ".provenance.json");
+    }
 }