feat(prune): R1 prune-reconcile pure core — (owned ∩ present) − referenced

Add REAPER-free/filesystem-free prune orphan-set core + additive
BankBook::referencedPaths() union query, with full unit coverage.

src/prune_reconcile.cpp

@@ -0,0 +1,36 @@
+#include "prune_reconcile.h"
+
+#include <unordered_set>
+
+// prune_reconcile implementation — the one set-algebra computation, kept trivially
+// auditable: build the referenced and owned lookup sets, then walk `present` once,
+// keeping a path iff it is owned AND not referenced. Walking `present` (not owned)
+// gives the ∩-present clause for free and yields output in folder-enumeration order.
+
+namespace reasampler {
+
+std::vector<std::string> pruneOrphans(const std::vector<std::string>& present,
+                                      const std::vector<std::string>& referenced,
+                                      const std::vector<std::string>& owned) {
+    // Exact-string membership — the model's canonical relative-path comparison
+    // (Sample.relativePath / OwnedFileManifest::contains). std::string hashes/compares
+    // byte-for-byte, so no normalization creeps in.
+    const std::unordered_set<std::string> referencedSet(referenced.begin(),
+                                                         referenced.end());
+    const std::unordered_set<std::string> ownedSet(owned.begin(), owned.end());
+
+    std::vector<std::string> orphans;
+    std::unordered_set<std::string> emitted;  // de-dup repeated spellings in `present`
+
+    for (const std::string& path : present) {
+        // (owned ∩ present) − referenced: present is the walk; owned and !referenced
+        // are the two membership tests; emitted guards against a duplicate `present`.
+        if (ownedSet.count(path) == 0) continue;         // not our leaving — skip
+        if (referencedSet.count(path) != 0) continue;    // some bank references it
+        if (!emitted.insert(path).second) continue;      // already emitted
+        orphans.push_back(path);
+    }
+    return orphans;
+}
+
+} // namespace reasampler