feat(actions): item-level Design/Arrange move + untag actions; W3-A polish

Add three MIDI-bindable item actions mirroring the track tag family, routed
through the shared hookcommand: retag selected items' membership then re-drive
mint/apply so each lands on its mode's managed lane (manual-lane items exempt),
all in one undo block. Extract shared item_read seam, simplify applyMintPlan's
lane-count pass, and guard reconcile against unregistered-mode lanes.

src/item_read.h

@@ -0,0 +1,34 @@
+#pragma once
+// item_read — the ONE place a MediaItem* is read for its canonical GUID string and for
+// the durable P_LANENAME of the fixed lane it sits on. Before this seam, view.cpp and
+// bank_panel.cpp each carried a near-identical private itemGuid / itemLaneName pair
+// (both files' comments acknowledged the deliberate copy); the D2 Wave-3-B item actions
+// need the same two reads, so the duplication is extracted here — the item-read analog
+// of track_guid's single MediaTrack* -> GUID-key formatter.
+//
+// REAPER-facing shell: the .cpp includes reaper_plugin_functions.h WITHOUT
+// REAPERAPI_IMPLEMENT (main.cpp owns the API pointers; here they are extern —
+// CLAUDE.md §contract). MediaItem / MediaTrack are forward-declared so this header
+// stays SDK-lite. These are shell reads (REAPER string/value getters); the managed/
+// manual DECISION that consumes the lane name stays pure in lane_keys (isOnManualLane).
+
+#include <string>
+
+class MediaItem;
+class MediaTrack;
+
+namespace reasampler {
+
+// An item's canonical GUID string via GetSetMediaItemInfo_String("GUID"). Empty on a
+// read failure (an empty GUID must never be tagged — every caller skips empties).
+std::string itemGuid(MediaItem* it);
+
+// The durable P_LANENAME of the fixed lane item `it` currently sits on (read via the
+// item's I_FIXEDLANE ordinal, then P_LANENAME:n on `tr`). Empty if the lane is unnamed
+// or the param is unavailable. Callers must already know `tr` is a fixed-lane track
+// (I_FREEMODE==2) before calling — I_FIXEDLANE is meaningless otherwise; the pure
+// isOnManualLane predicate handles the non-fixed-lane case via its own argument, so
+// callers should not call this at all for a normal track.
+std::string itemLaneName(MediaTrack* tr, MediaItem* it);
+
+}  // namespace reasampler