feat: send text to named sessions with draft protection and listener isolation

- chat send and listener-smoke accept --session; defaults remain File Transfer Assistant.
- Reject existing drafts, reacquire the send control after input changes, and require a
  fresh message fingerprint for confirmation without retrying failed sends.
- Test-NamedTextSend sends the same text twice and verifies distinct confirmed fingerprints;
  Test-NamedListeners -SendMarkers verifies per-window own events exactly once with zero
  cross-session events. Note export now requires a unique fresh owned note window and keeps
  repeated paragraphs. 97 Core tests pass; real-machine validated.

src/WxAgent.Core/WechatNoteWindow.cs

@@ -0,0 +1,16 @@
+namespace WxAgent.Core;
+
+public sealed record WechatNoteWindowCandidate(long Handle, int ProcessId, string Title, bool IsVisible, bool HasChatSurface);
+
+public static class WechatNoteWindow
+{
+    public static long? FindOpened(IEnumerable<WechatNoteWindowCandidate> windows, int processId, IReadOnlySet<long> previousHandles)
+    {
+        var candidates = windows.Where(window => window.Handle != 0 && window.ProcessId == processId
+            && !previousHandles.Contains(window.Handle) && window.IsVisible && !window.HasChatSurface
+            && window.Title.Contains("笔记", StringComparison.Ordinal)).ToArray();
+        if (candidates.Length > 1)
+            throw new WxAgentException(WxAgentErrorCode.UiStructureChanged, "Multiple new note windows appeared; refusing ambiguous content.");
+        return candidates.Length == 1 ? candidates[0].Handle : null;
+    }
+}