v1.8.36: fix fullscreen toggle — _is_detached flag prevents re-embed conflict

- Added _is_detached flag to EmbeddedRDP (set in detach(), cleared in reattach())
- _monitor_tick() skips try_reembed() when window is intentionally detached
- Prevents race condition: monitor tick no longer re-embeds fullscreen window
- Added plans/test-rdp.md — RDP testing documentation
- build.py auto-cleanup removed v1.8.31

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/remote_desktop.py

@@ -243,6 +243,7 @@ class EmbeddedRDP:
         self._rdp_file: str | None = None
         self._connected = False
         self._parent_hwnd: int | None = None
+        self._is_detached = False  # True when intentionally fullscreen
 
         # Callbacks (set by GUI)
         self.on_embedded = None    # called when window is embedded
@@ -612,6 +613,7 @@ class EmbeddedRDP:
         """Detach mstsc from parent — for fullscreen."""
         if not self._mstsc_hwnd or not self._connected:
             return
+        self._is_detached = True
         try:
             import ctypes
             import ctypes.wintypes
@@ -644,6 +646,7 @@ class EmbeddedRDP:
         """Re-embed mstsc back into the tkinter frame."""
         if not self._mstsc_hwnd:
             return
+        self._is_detached = False
         self._embed(parent_hwnd, width, height)
         log.info("mstsc reattached")