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.h

@@ -58,6 +58,14 @@ std::optional<std::string> managedLaneKey(const std::string& laneName);
 // contract is asserted here so minting and reading cannot drift.
 std::string laneNameForMode(const std::string& modeId);
 
+// The owning mode id encoded in a managed lane NAME — the suffix after the managed
+// prefix. std::nullopt for a manual/unnamed lane (no managed prefix) or a name that is
+// EXACTLY the prefix with no mode suffix (illegal — a managed lane always names a mode).
+// The exact inverse of laneNameForMode: modeIdFromLaneName(laneNameForMode(m)) == m.
+// Used by the load-time reconcile to recover managed ownership from REAPER's durable
+// lane name (the source of truth for identity across sessions — design point #2).
+std::optional<std::string> modeIdFromLaneName(const std::string& laneName);
+
 // True iff an item on a fixed-lane track with the given lane name is on a MANUAL lane
 // (i.e. exempt from auto-tag). The two inputs are:
 //   isFixedLaneTrack — whether the item's track has I_FREEMODE==2. On a normal