Ψ-W1-T2 review remediation: solo restore drops on visible-in-target, not parked; N-mode segments read dead when unroutable

Fixes a hidden-parent solo replay that could silence the mix. Also closes the N-mode segment silent no-op, amends the invariant comment, trims view.cpp under 600 lines, hedges two SDK inferences, drops a dead null-check.

src/shell/view/CLAUDE.md

@@ -83,7 +83,7 @@ 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` + per-FX offline), 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_solo` — the `I_SOLO` read/write pair behind the per-mode solo surface. Holds no policy: what to cache, clear, or replay is `core/view/solo_cache`.
+- `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
 
@@ -94,3 +94,9 @@ applies the resulting lane state to live tracks.
   (fixed-lane mechanics, mode-aware capture placement, hidden-AND-silenced) are
   reflected in Invariants above; the membership/lane-ownership model concepts
   it also covers live in `core/view`'s Invariants.
+- REAPER's own undo restores live `I_SOLO` but not the model's solo cache or
+  `activeModeId` — neither rolls back with a Ctrl-Z. An undo after a mode switch
+  leaves the two out of step, and the next switch banks the undo-restored solos
+  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.