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.cpp

@@ -0,0 +1,33 @@
+// item_read.cpp — the single MediaItem* read seam (GUID + fixed-lane name). See
+// item_read.h. Compiled into the reaper_reasampler MODULE; includes
+// reaper_plugin_functions.h WITHOUT REAPERAPI_IMPLEMENT (main.cpp is the one TU that
+// defines the API pointers — CLAUDE.md §contract).
+
+#include "item_read.h"
+
+#include <cstdio>
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_GetSetMediaItemInfo_String
+#define REAPERAPI_WANT_GetMediaItemInfo_Value
+#define REAPERAPI_WANT_GetSetMediaTrackInfo_String
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+std::string itemGuid(MediaItem* it) {
+    char buf[64] = {0};
+    if (!GetSetMediaItemInfo_String(it, "GUID", buf, false)) return {};
+    return std::string(buf);
+}
+
+std::string itemLaneName(MediaTrack* tr, MediaItem* it) {
+    const int laneIdx = static_cast<int>(GetMediaItemInfo_Value(it, "I_FIXEDLANE"));
+    char parm[32];
+    std::snprintf(parm, sizeof(parm), "P_LANENAME:%d", laneIdx);
+    char buf[512] = {0};
+    if (!GetSetMediaTrackInfo_String(tr, parm, buf, false)) return {};
+    return std::string(buf);
+}
+
+}  // namespace reasampler