Docs: record plugin-editor drop; update SEPT_VERIFY plugin-drop checks

docs/COMPLETED.md

@@ -1793,3 +1793,21 @@ in the same block as a note-on failed to reach the already-held, sounding voices
 voices (`VoiceEngine::startVoice` now calls `applyLiveToActive()`); and the amp spline's read is
 clamped to `[0,1]` to absorb a swap-step overshoot from the smoother. Not DAW-verified —
 checks are tracked in `docs/SEPT_VERIFY.md` §5.
+
+### Plugin-editor drop hand-off
+
+Dragging a bank card onto a third-party plugin's own editor window (e.g. Serum 2's
+oscillator) now hands the file to Windows as an ordinary OS file drop, so the plugin
+loads it — the same outcome as dragging the file in from Explorer. REAPER's other
+surfaces keep their existing outcomes: dropping on the FX chain list or FX window
+chrome still loads ReaSampler 9000, and TCP/MCP/arrange are unchanged. ReaSampler
+9000's own editor is exempted from the hand-off (`WindowFacts::ownModule`, a basename
+match against the channel's own `.vst3`) — a drop on it still loads the instrument.
+The OS hand-off is reversible mid-drag: moving back over REAPER (including after
+leaving REAPER for Explorer) hands the drag back to the panel's own drop behavior.
+Stock REAPER plugins (ReaEQ etc.) now count as plugin editors for this purpose. Landed
+in the same merge: stable and beta ReaSampler 9000 editors previously shared one
+Windows window class, so side-by-side installs could run the wrong channel's code —
+each channel now registers its own class. Mechanism detail: `src/core/ui/CLAUDE.md`
+(`window_origin`) and `src/shell/actions/drag_out_win.h`. Not DAW-verified — checks
+are tracked in `docs/SEPT_VERIFY.md` §4.

docs/SEPT_VERIFY.md

@@ -51,7 +51,7 @@ project rate 48000, unless a check says otherwise.
 - [ ] For mode-switching itself (solo-set restore, transport refusal, persistence, undo point count,
   etc.) — see `docs/VERIFICATION.md` §"Mode switching"; not repeated here
 
-## 4. Drag a card into a plugin's editor (in progress, Sept 2026)
+## 4. Drag a card into a plugin's editor (landed Sept 2026)
 
 - [ ] Drop a bank card on Serum 2's oscillator in its floating FX editor window — Serum loads the
   sample
@@ -65,9 +65,14 @@ project rate 48000, unless a check says otherwise.
   change)
 - [ ] Drag a card out to Windows Explorer and back into REAPER — control hands back to the panel's
   own drag correctly
-- [ ] Drop a card onto an already-open ReaSampler 9000 editor — it shows its "drop on the panel"
+- [ ] Drop a card onto an already-open ReaSampler 9000 editor — it loads ReaSampler 9000 onto the
-  banner and does NOT spawn a second instance
+  track (no banner, no OS drop)
 - [ ] Drop a card onto ReaEQ's editor — shows a no-drop cursor, nothing loads
+- [ ] Drop a card onto a plugin editor that doesn't accept files (e.g. a compressor) — note whether
+  it is refused (no-drop cursor, nothing loads) or lands in REAPER's own file import; either outcome
+  is accepted, but record which one actually happens
+- [ ] Run stable and beta ReaSampler 9000 side by side and open both editors — each works normally,
+  no crash
 
 ## 5. Live spline-envelope drags (in progress, Sept 2026)