feat(view): D2 Wave 2 — apply managed-lane ops + timer-diff auto-tag new content

Drives fixed-lane C_LANEPLAYS for managed lanes only (name-keyed, ordinal-renumber
safe) in the view shell; diffs live track/item GUIDs on the panel timer to auto-tag
new content to the active mode, first-poll-guarded. Pure guid_diff + lane_keys
modules unit-tested; folds in Wave-1 polish.

src/guid_diff.cpp

@@ -0,0 +1,44 @@
+// guid_diff implementation — pure set arithmetic for new-content detection. See
+// guid_diff.h. No REAPER, no SWELL — std only.
+
+#include "guid_diff.h"
+
+#include <algorithm>
+
+namespace reasampler {
+
+std::vector<std::string> newGuids(const std::set<std::string>& previous,
+                                  const std::set<std::string>& current) {
+    std::vector<std::string> added;
+    // current \ previous. std::set iterates ascending, so set_difference yields a
+    // deterministic order without a separate sort.
+    for (const std::string& g : current) {
+        if (g.empty()) continue;             // never tag a GUID-read failure
+        if (previous.count(g) == 0) added.push_back(g);
+    }
+    return added;
+}
+
+std::vector<std::string> GuidBaseline::observe(const std::set<std::string>& current) {
+    if (!primed_) {
+        // First poll after open/reset: establish the baseline, report nothing new so
+        // pre-existing content is NOT auto-tagged (it defaults to Arrange).
+        baseline_ = current;
+        primed_ = true;
+        return {};
+    }
+    std::vector<std::string> added = newGuids(baseline_, current);
+    // Advance the baseline to the full current set. Using `current` (not baseline_ ∪
+    // added) means a DELETED GUID drops out of the baseline too, so if REAPER later
+    // reuses that GUID for genuinely new content it is detected again — the baseline
+    // tracks the live set exactly, not a monotonic union.
+    baseline_ = current;
+    return added;
+}
+
+void GuidBaseline::reset() {
+    baseline_.clear();
+    primed_ = false;  // next observe() re-baselines (first-poll guard re-armed)
+}
+
+}  // namespace reasampler