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/core/view/solo_cache.h

@@ -73,14 +73,18 @@ private:
 // The incoming mode's restore writes, in GUID order. Two kinds of entry are dropped
 // rather than written:
 //   * a GUID absent from `liveGuids` — the track is gone (same prune rule as above);
-//   * a GUID parked in the incoming mode — a parked track is hidden and carries
-//     B_MAINSEND=0, so soloing it would silence the whole mix while contributing
-//     nothing audible, and the user would have no visible control to undo it.
+//   * a GUID absent from `visibleGuids` — not visible in the incoming mode, whether
+//     because the leaf is parked or because it is a folder parent that is itself
+//     derived-invisible there. Either way the track is hidden and (for a parked
+//     leaf) carries B_MAINSEND=0, so soloing it would silence the whole mix while
+//     contributing nothing audible, and the user would have no visible control to
+//     undo it. A show-both leaf is a member of every mode's visible set, so it is
+//     never dropped by this rule.
 // The caller consumes the whole mode entry regardless (clear-on-restore), so a
 // dropped entry does not linger as zombie state waiting on a track that may never
-// come back unparked.
+// become visible again.
 std::vector<SoloOp> planSoloRestore(const std::map<std::string, int>& cached,
                                     const std::set<std::string>& liveGuids,
-                                    const std::set<std::string>& parkedGuids);
+                                    const std::set<std::string>& visibleGuids);
 
 } // namespace reasampler::view