feat(view): mint managed lanes to separate cross-mode content per stance (D2 W3-A)

Pure planLaneMinting decides which tracks hold >1 mode's content and which managed
lane each item lands on; view.cpp applies it (I_FREEMODE/I_NUMFIXEDLANES/P_LANENAME/
I_FIXEDLANE) under one undo block, driven off the auto-tag detection tick. Manual lanes
and their items are never touched. Load-time reconcile rebuilds ownership from durable
lane names before reapplying active-mode visibility.

tests/test_lane_keys.cpp

@@ -82,11 +82,31 @@ static void testRoundTrip() {
     }
 }
 
+static void testModeIdFromLaneName() {
+    // The exact inverse of laneNameForMode: recover the owning mode from a managed name.
+    // Used by the Wave-3 load-time reconcile to rebuild ownership from durable names.
+    for (const std::string mode : {std::string("arrange"), std::string("design"),
+                                   std::string("mixdown"), std::string("mode:with:colons")}) {
+        auto recovered = modeIdFromLaneName(laneNameForMode(mode));
+        CHECK(recovered.has_value() && *recovered == mode);  // modeIdFromLaneName∘laneNameForMode == id
+    }
+
+    // Manual / unnamed lanes carry no mode (⇒ left off the ownership index on reconcile).
+    CHECK(!modeIdFromLaneName("").has_value());
+    CHECK(!modeIdFromLaneName("Comp 1").has_value());
+    CHECK(!modeIdFromLaneName("Reasampler:design").has_value());  // wrong case ⇒ manual
+
+    // Prefix-only with no mode suffix is illegal for a managed lane ⇒ no mode recovered
+    // (defensive: reconcile skips it rather than recording an empty-mode ownership).
+    CHECK(!modeIdFromLaneName("reasampler:").has_value());
+}
+
 int main() {
     testIsManagedLaneName();
     testManagedLaneKey();
     testIsOnManualLane();
     testRoundTrip();
+    testModeIdFromLaneName();
 
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;