fix(D2-W2): correct manual-lane exemption, drop read-only item C_LANEPLAYS write

isOnManualLane predicate now correctly treats unnamed fixed-lane lanes as manual;
item-side C_LANEPLAYS loop removed (SDK read-only); explicit #include <set> in
view_mode_model.cpp; undo-block omission in detectNewContent documented as intentional.

src/lane_keys.cpp

@@ -31,4 +31,14 @@ std::string laneNameForMode(const std::string& modeId) {
     return std::string(kManagedLanePrefix) + modeId;
 }
 
+bool isOnManualLane(bool isFixedLaneTrack, const std::string& laneName) {
+    // On a normal (non-fixed-lane) track there is no concept of a manual lane; the
+    // item follows the normal auto-tag rule.
+    if (!isFixedLaneTrack) return false;
+    // On a fixed-lane track: a managed lane (prefixed) is NOT manual; everything else
+    // — including the empty/unnamed lane that REAPER creates by default — IS manual
+    // (user-minted, off-limits to auto-tag and to the lane-drive path).
+    return !hasManagedPrefix(laneName);
+}
+
 }  // namespace reasampler