ci: observe transcript append failure and Windows reader sharing

tools/ci/TranscriptFileShareProbe.cs

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+#:property PublishAot=false
+// Local diagnostic only. See TranscriptFileShareProbe.md for the contract and limits.
+using System.Diagnostics;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Text.Json;
+
+const string original = "---\ntitle: transcript probe\n---\n\n# Meeting Transcript\n";
+const string written = original + "[00:00:04] Guest-1: Azure returned ***** here.\n";
+var root = Path.Combine(Path.GetTempPath(), "meeting-assistant-file-share-probe", Guid.NewGuid().ToString("N"));
+if (Directory.Exists(root))
+    throw new IOException("Probe directory already exists; refusing to reuse it.");
+Directory.CreateDirectory(root);
+var cases = new List<WriteObservation>();
+var cleanupCompleted = false;
+try
+{
+    var path = Path.Combine(root, "original-reader.md");
+    await File.WriteAllTextAsync(path, original);
+    using (var reader = new StreamReader(path, Encoding.UTF8, detectEncodingFromByteOrderMarks: true))
+    {
+        if (reader.ReadToEnd() != original)
+            throw new InvalidDataException("Original reader did not read the initial fixture.");
+        cases.Add(await ObserveWriteAsync("held-original-reader", path, written));
+    }
+    cases[^1] = cases[^1] with { ContentAfterReaderClosed = await File.ReadAllTextAsync(path) };
+    cases.Add(await ObserveWriteAsync("original-reader-released", path, written));
+    cases[^1] = cases[^1] with { ContentAfterReaderClosed = await File.ReadAllTextAsync(path) };
+
+    path = Path.Combine(root, "compatible-reader.md");
+    await File.WriteAllTextAsync(path, original);
+    using (var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete))
+    using (var reader = new StreamReader(stream, Encoding.UTF8, detectEncodingFromByteOrderMarks: true))
+    {
+        if (reader.ReadToEnd() != original)
+            throw new InvalidDataException("Compatible reader did not read the initial fixture.");
+        cases.Add(await ObserveWriteAsync("held-compatible-reader", path, written));
+    }
+    cases[^1] = cases[^1] with { ContentAfterReaderClosed = await File.ReadAllTextAsync(path) };
+}
+finally
+{
+    Directory.Delete(root, recursive: true);
+    cleanupCompleted = !Directory.Exists(root);
+}
+
+var windowsContractMatched = OperatingSystem.IsWindows()
+    ? !cases[0].Completed && cases[0].Error is { Type: "System.IO.IOException", NativeCode: 32 }
+        && cases[0].ContentAfterReaderClosed == original
+    : (bool?)null;
+var compatibleAndReleasedWritesCompleted = cases.Skip(1).All(result =>
+    result.Completed && result.Error is null && result.ContentAfterReaderClosed == written);
+Console.WriteLine(JsonSerializer.Serialize(new
+{
+    Schema = "meeting-assistant-file-share-probe/v1",
+    Runtime = RuntimeInformation.FrameworkDescription,
+    OS = RuntimeInformation.OSDescription,
+    Architecture = RuntimeInformation.ProcessArchitecture.ToString(),
+    ProbeDirectory = root,
+    CleanupCompleted = cleanupCompleted,
+    WindowsContractMatched = windowsContractMatched,
+    CompatibleAndReleasedWritesCompleted = compatibleAndReleasedWritesCompleted,
+    Cases = cases,
+    EvidenceLimit = "Held-reader access-mode probe; it does not reproduce the timing or establish the cause of Run4174."
+}, new JsonSerializerOptions { WriteIndented = true }));
+return cleanupCompleted && compatibleAndReleasedWritesCompleted && windowsContractMatched != false ? 0 : 1;
+
+static async Task<WriteObservation> ObserveWriteAsync(string name, string path, string content)
+{
+    var elapsed = Stopwatch.StartNew();
+    Task? write = null;
+    using var deadline = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+    try
+    {
+        write = File.WriteAllTextAsync(path, content, deadline.Token);
+        await write;
+        return new(name, true, write.Status.ToString(), elapsed.ElapsedMilliseconds, null, null);
+    }
+    catch (Exception exception)
+    {
+        return new(name, false, write?.Status.ToString() ?? "not-returned", elapsed.ElapsedMilliseconds,
+            new(exception.GetType().FullName!, $"0x{exception.HResult:X8}", exception.HResult & 0xffff, exception.Message), null);
+    }
+}
+
+sealed record WriteObservation(string Name, bool Completed, string TaskStatus, long ElapsedMilliseconds,
+    WriteError? Error, string? ContentAfterReaderClosed);
+sealed record WriteError(string Type, string HResult, int NativeCode, string Message);

tools/ci/TranscriptFileShareProbe.md

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+# Transcript file sharing diagnostic
+
+Purpose: distinguish a writer error from a completed write when a reader with the original test's access mode remains open. This temporary diagnostic does not change the app, its tests, or their 577-case count.
+
+Entry point: `tools/ci/TranscriptFileShareProbe.cs`, a .NET 10 file-based app with BCL-only dependencies. From this clone:
+
+```sh
+/Users/dh/.dotnet/dotnet run --file tools/ci/TranscriptFileShareProbe.cs
+```
+
+On another machine use its .NET 10 SDK executable. The file-based app requires an SDK supporting file-based apps; the prepared local run uses SDK 10.0.401. Native AOT is disabled for this diagnostic so its JSON report can use the normal reflection serializer. It prints JSON with runtime/OS, write completion, exception type/HResult/native error code, content after reader disposal, and cleanup status. It creates a unique directory beneath the system temporary directory, writes two small fixture files, and deletes only that directory in `finally`. It starts no Meeting Assistant app, service, network client, container, or VM. The SDK can create its normal file-based build cache. For a fresh CLI profile set `DOTNET_GENERATE_ASPNET_CERTIFICATE=false` and `DOTNET_CLI_TELEMETRY_OPTOUT=1` to disable unrelated certificate/telemetry initialization.
+
+The optional instrumentation in the existing recording-coordinator test observes public provider and transcript-store boundaries: audio consumed, fake segment yielded, append/rewrite entered, completed, or failed. Timeout output uses `[DEBUG-transcript-write-4174]` and includes elapsed milliseconds, exception type, HResult, and message. The reader, 15-second wait and final redaction assertions are unchanged. `StopAsync` always runs in `finally`; a stop failure does not mask the original timeout. Timestamps distinguish events before and after the failed wait. Instrumentation can affect race timing; a passing run alone does not explain the original failure. This temporary instrumentation should be removed after the actual Wine incident is explained.
+
+The original-reader case uses the same path-taking `StreamReader` constructor as `File.ReadAllText`. It deliberately holds the reader after reading the fixture so that the overlap is deterministic. The original test normally disposes that reader immediately after `ReadToEnd`; therefore this probe checks compatible access modes, not the historical race's timing. The second write happens after disposing that reader. The compatible case holds `FileAccess.Read` with `FileShare.ReadWrite | FileShare.Delete`. No reader fix is applied to the actual test.
+
+The existing Wine job runs this probe after its actual Windows SDK build and before the unchanged full test cohort. It writes `artifacts/tests/transcript-file-sharing.json` and prints that report into the CI log. Invocation there uses the already installed Windows SDK through the existing `WINE_BIN`; no runner or infrastructure capability is added. The measured Wine result is pending until this exact workflow executes.
+
+## Primary-source contract
+
+In [.NET runtime v10.0.12 File.cs](https://github.com/dotnet/runtime/blob/v10.0.12/src/libraries/System.Private.CoreLib/src/System/IO/File.cs#L572), `ReadAllText` constructs a path-taking `StreamReader`; [`StreamReader.cs`](https://github.com/dotnet/runtime/blob/v10.0.12/src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs#L203) opens read access sharing only further readers. `WriteAllTextAsync` delegates to a create-mode write; [its writer](https://github.com/dotnet/runtime/blob/v10.0.12/src/libraries/System.Private.CoreLib/src/System/IO/File.cs#L1416) opens write access with reader sharing.
+
+[Windows CreateFileW documentation](https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilew#parameters) requires existing access and sharing modes to remain compatible until handle closure. A held reader that does not permit writes therefore prevents that writer from opening; the Windows contract expects an `IOException` with sharing-violation native code 32. Allowing read/write sharing removes that incompatibility. Delete sharing is included for the comparison but this probe does not rename or delete an open file.
+
+On Windows the CLI asserts that the original held-reader write fails with code 32 and leaves the original bytes, and that both subsequent writes complete with the expected content. On other platforms it reports the original-reader observation without asserting Windows behavior (`WindowsContractMatched: null`), and still checks completed compatible/released writes and cleanup. The Unix/macOS implementation can differ. A local macOS success is not evidence of Wine behavior or the cause of Run4174's timeout.