Clear the park snapshot where the restore is planned, not where it drains; make the drain re-entrant and reload-aware

src/shell/view/CLAUDE.md

@@ -55,6 +55,13 @@ decide membership or mode rules.
   the switch's undo block, so the tool no longer re-drives them on an undo or a
   redo — what a Ctrl-Z then leaves the chain at is REAPER's own FX-state record,
   `[verify — DAW]`.
+- **Stated DEVIATION — the undo mask does not keep FX out of a real switch.** The
+  apply mask (`kApplyUndoMask`) drops `UNDO_STATE_FX` and ORs it back in when a
+  driven flag in that domain moved; the only such flag is `I_FXEN`, which every
+  park writes. So any switch that parks at least one track still makes REAPER
+  marshal the project-wide FX chunk into its undo record. The saving is real only
+  on a reapply and on a no-op switch. Narrowing it further would mean not carrying
+  `I_FXEN` in the undo record at all, which would break "one switch is one Ctrl-Z."
 - **Show-both semantics:** a per-track "pin visible across modes" flag re-enables
   processing whenever shown. A show-both leaf appears in every mode's visible set
   and is never parked — its driven flags stay at snapshot/restored values, FX
@@ -93,7 +100,7 @@ applies the resulting lane state to live tracks.
 ## Modules
 
 - `view` — Design View shell: snapshots flag values before parking, drives hide + CPU-park on inactive-mode leaves (`B_SHOWINTCP`/`B_SHOWINMIXER`/`B_MAINSEND`/`I_FXEN`, with per-FX offline deferred to `view_fx_park`), restores from snapshot. Owns the one discriminator (`target != active`) that separates a real switch from a reapply, and with it both the playback gate (`transportBlocksModeSwitch`) and the solo cache/clear/restore seams. **Never touches master or `B_MUTE`.**
-- `view_fx_park` — the per-FX offline surface: the `TrackFX_GetFXGUID` identity read snapshot/park/restore share, the deferred intent queue that keeps `TrackFX_SetOffline` off the switch's synchronous path (at most one intent per track GUID, latest wins, an intent landing on its own pending inverse cancels it), and the idle-tick drain `main.cpp`'s `OnTimer` calls. The drain is also where a consumed snapshot is cleared — not where the restore was planned — so a second switch arriving before the first drained re-parks against the still-true captured state instead of re-capturing parked values.
+- `view_fx_park` — the per-FX offline surface: the `TrackFX_GetFXGUID` identity read snapshot/park/restore share, the deferred intent queue that keeps `TrackFX_SetOffline` off the switch's synchronous path (at most one intent per track GUID, latest wins, an intent landing on its own pending inverse cancels it), and the idle-tick drain `main.cpp`'s `OnTimer` calls. **The drain owns no model state.** A snapshot is dropped where the restore is PLANNED — the flags are back at their captured values from that moment, and a model that still described the track as parked would let a persist or a reapply inside the drain window replan a restore over whatever the user changed since. What the deferral costs instead is that the live FX chain stops being a trustworthy snapshot source while an intent is pending: a park that CANCELS a pending restore takes the pre-park FX states from that restore's ops (`preParkFxFromCancelledRestore`), because the chain still reads the parked values and the cancel means no drain will ever fix them.
 - `view_solo` — the `I_SOLO` read/write pair behind the per-mode solo surface, plus `clearTrackSolos`/`restoreTrackSolos`, the outgoing-clear and incoming-replay entry points `view` drives them through. Holds no policy: what to cache, clear, or replay is `core/view/solo_cache`.
 
 ## Gotchas