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.

src/lane_keys.cpp

@@ -31,6 +31,13 @@ std::string laneNameForMode(const std::string& modeId) {
     return std::string(kManagedLanePrefix) + modeId;
 }
 
+std::optional<std::string> modeIdFromLaneName(const std::string& laneName) {
+    if (!hasManagedPrefix(laneName)) return std::nullopt;  // manual/unnamed ⇒ no mode
+    const std::size_t n = std::strlen(kManagedLanePrefix);
+    if (laneName.size() == n) return std::nullopt;  // prefix only, no mode suffix (illegal)
+    return laneName.substr(n);
+}
+
 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.