Fix stuck drag after OS hand-off withdrawal, split panel_drag_os, doc fixes

Re-checks the physical button before resuming an internal drag and treats a button-up move mid-drag as a release, closing a stale-payload/silent-drop bug; splits the OS hand-off seam into its own TU for the line ceiling; corrects two docs and skips a redundant module-path lookup.

src/shell/actions/drag_out_win.h

@@ -64,9 +64,21 @@ bool pointerOverHostWindow(int screenX, int screenY);
 //   - a plugin that draws straight into the parent REAPER hands it, creating no window of its
 //     own, leaves nothing of its code in the chain;
 //   - a plugin binary installed beneath the Windows directory reads as an OS module;
-//   - a class procedure that resolves to no module (a charset-mismatch handle) is no evidence.
+//   - a class procedure that resolves to no module (a charset-mismatch handle) is no evidence;
+//   - an all-#32770 dialog-based editor (the generic Windows dialog class) resolves to no
+//     recognizable plugin module — falls to REAPER's outcome;
+//   - a .NET/WPF editor's class procedure is a CLR-thunked address with no resolvable module —
+//     also falls to REAPER's outcome.
+// The opposite direction is unguarded: a third-party hook that replaces REAPER's own chrome
+// class procedure via SetClassLongPtr(GCLP_WNDPROC) would make REAPER's own window read as
+// Plugin — toward OLE instead of away from it.
 // [verify — DAW] that REAPER's GetThingFromPoint names an fx_* token over the plugin's own child
 // window — the law requires it, so an unnamed editor would refuse rather than hand off.
+// [verify — DAW] that OLE never walks up to an ancestor drop target (e.g. REAPER's own file
+// import) when the plugin editor's window registers no IDropTarget of its own.
+// [verify — DAW] that GetThingFromPoint cannot name a REAPER region (e.g. an fx_* token) when a
+// foreign window overlaps it at the same screen point — this fn trusts WindowFromPoint's hit
+// over whatever GetThingFromPoint separately reports.
 // SWELL: always false (no plugin-editor hand-off off Windows). Side-effect-free.
 bool pointerOverPluginEditor(int screenX, int screenY);