fix(audience): narrow BuildPayload catch to IOException

Addresses review feedback on PR #691. The previous catch (Exception) in
BuildPayload swallowed every failure as if it were the expected
file-disappeared race. Permissions errors (UnauthorizedAccessException)
and similar non-IO faults were silently dropped, masking real problems.

Narrowing to IOException keeps the file-disappeared / locked / path-
gone cases skipped — which is what the retry loop needs — while letting
genuinely broken states propagate.

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

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Transport/HttpTransport.cs

@@ -192,9 +192,11 @@ private static string BuildPayload(IReadOnlyList<string> paths)
                     sb.Append(json);
                     count++;
                 }
-                catch (Exception)
+                catch (IOException)
                 {
-                    // File disappeared between ReadBatch and now — skip it.
+                    // File disappeared, locked, or path vanished between ReadBatch
+                    // and now — skip it. Non-IO errors like UnauthorizedAccessException
+                    // indicate real problems and are allowed to propagate.
                 }
             }