feat(prune): R3 guarded deletion — confirm-to-delete + panel button

Extend BANK_PRUNE_FOLDER from report-only to dry-run→confirm-with-manifest→
delete exactly the pure-core orphan set (fresh recompute, stale entries skipped).
Windows routes to Recycle Bin (SHFileOperation+FOF_ALLOWUNDO), else unlink.
New pure prune_button module + footer button dispatching the action id. No
ext-state write, no undo point (deletion is not REAPER-undoable).

src/prune_reconcile.cpp

@@ -52,4 +52,21 @@ PruneReport buildPruneReport(
     return report;
 }
 
+std::vector<std::string> pruneDeletePlan(const std::vector<std::string>& confirmed,
+                                         const std::vector<std::string>& freshOrphans) {
+    // fresh is a pure-core output (referenced/hand-dropped already excluded), so keeping a
+    // confirmed path iff it is still a fresh orphan can never re-admit an unsafe file. Walk
+    // `confirmed` to preserve the confirm's listing order; emitted de-dups repeats.
+    const std::unordered_set<std::string> freshSet(freshOrphans.begin(), freshOrphans.end());
+
+    std::vector<std::string> plan;
+    std::unordered_set<std::string> emitted;
+    for (const std::string& path : confirmed) {
+        if (freshSet.count(path) == 0) continue;    // stale: vanished / now-referenced -> skip
+        if (!emitted.insert(path).second) continue; // already emitted
+        plan.push_back(path);
+    }
+    return plan;
+}
+
 } // namespace reasampler