Drain deferred FX parks before the view model is serialized; re-validate track handles per intent; pin the restore-plan round trip

src/shell/view/CLAUDE.md

@@ -54,7 +54,14 @@ decide membership or mode rules.
   lost. What it does change is the undo record: the offline writes land outside
   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]`.
+  `[verify — DAW]`. **The idle tick is not the only drain point.** Any path that
+  serializes the view model drains synchronously first (`persistViewState`,
+  `render_in_place`), because a save landing between a restore's synchronous flag
+  writes and its drain would record offline FX beside a model that no longer
+  carries the snapshot to replan them — unrecoverable on reopen. So an
+  action-driven switch does pay the FX hitch before it returns; the repaint and
+  the undo block have both closed by then, which is what the deferral was for.
+  Any new caller that reapplies a mode and then persists inherits this obligation.
 - **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
@@ -118,6 +125,12 @@ applies the resulting lane state to live tracks.
   under whatever mode id is active at that point, not the one the user undid back
   to. Pre-existing: `snapshots_` already carries this same model-vs-undo split;
   the solo cache inherits it rather than introducing it. Not fixed here.
+- An undo/redo also DISCARDS every pending FX intent (`discardDeferredFxParks`),
+  which is not the pure loss it reads as: the same tick reapplies the active mode
+  over the reloaded model, re-planning a park for every inactive leaf, so parked
+  FX converge on the following drain. The one case that does not self-heal is a
+  track whose reloaded model carries no snapshot — nothing plans its restore, so
+  FX left offline stay offline. Full contract at `discardDeferredFxParks`.
 - `fx_offline`'s identity keying (`TrackFX_GetFXGUID`) assumes the GUID stays
   attached to its plugin across a chain mutation while parked. That is
   `[verify — DAW]` (see `fxGuidString` in `view_fx_park.cpp`) and SWS issue #802 is a