feat(view): D2 Wave 2 — apply managed-lane ops + timer-diff auto-tag new content

Drives fixed-lane C_LANEPLAYS for managed lanes only (name-keyed, ordinal-renumber
safe) in the view shell; diffs live track/item GUIDs on the panel timer to auto-tag
new content to the active mode, first-poll-guarded. Pure guid_diff + lane_keys
modules unit-tested; folds in Wave-1 polish.

tests/test_view_mode_model.cpp

@@ -940,6 +940,40 @@ static void testLaneOwnershipIndex() {
     CHECK(!idx.remove("{T}", "l0"));
 }
 
+// -- D2.2b Last-writer-wins ownership replace (round-trip) --------------------
+//
+// setManual then setManaged on the SAME (guid, laneKey) must leave EXACTLY ONE
+// managed entry — the ownership record is replaced, not accumulated. Guards the
+// "retag a lane the tool now owns" contract and its persistence: the replace must
+// survive a serialize/deserialize round-trip with no stray manual duplicate.
+static void testLaneOwnershipLastWriterWins() {
+    ViewModeModel vm;
+
+    // Manual first, then managed on the same lane — the managed write replaces.
+    CHECK(vm.lanes().setManual("{T}", "l0"));
+    CHECK(vm.lanes().setManaged("{T}", "l0", kDesignModeId));
+    CHECK(vm.lanes().size() == 1);                       // one entry, not two
+    const LaneOwnership* o = vm.lanes().query("{T}", "l0");
+    CHECK(o && o->isManaged() && *o->managedMode == kDesignModeId);
+
+    // The reverse also replaces: managed -> manual leaves exactly one manual entry.
+    CHECK(vm.lanes().setManual("{T}", "l0"));
+    CHECK(vm.lanes().size() == 1);
+    const LaneOwnership* m = vm.lanes().query("{T}", "l0");
+    CHECK(m && m->isManual());
+
+    // Back to managed, then round-trip: exactly one managed entry survives, no stray
+    // manual duplicate resurrected by (de)serialization.
+    CHECK(vm.lanes().setManaged("{T}", "l0", kArrangeModeId));
+    auto back = ViewModeModel::deserialize(vm.serialize());
+    CHECK(back.has_value());
+    if (back) {
+        CHECK(back->lanes().size() == 1);
+        const LaneOwnership* r = back->lanes().query("{T}", "l0");
+        CHECK(r && r->isManaged() && *r->managedMode == kArrangeModeId);
+    }
+}
+
 // -- D2.3/D2.4 Managed-only: planner + query never emit a manual lane --------
 //
 // Required case: a track with a manual lane + managed mode lanes — neither the planner
@@ -1124,6 +1158,7 @@ int main() {
     // D2 two-canvas lane extension
     testLaneModeStateAndPlayValues();
     testLaneOwnershipIndex();
+    testLaneOwnershipLastWriterWins();
     testManagedOnlyPlannerAndQuery();
     testAutoTagDecision();
     testLaneJsonRoundTrip();