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/bank_book.cpp

@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <climits>
+#include <unordered_set>
 
 // bank_book implementation.
 //
@@ -321,6 +322,22 @@ bool BankBook::hashReferencedElsewhere(const std::string& hash,
     return false;
 }
 
+std::vector<std::string> BankBook::referencedPaths() const {
+    // Union across the whole book (pool first, then named banks in ordinal order —
+    // banks_ is kept ordinal-sorted). De-duplicate by exact string so a file a copy
+    // put in two banks appears once. Skip empty paths (they reference no file).
+    std::vector<std::string> paths;
+    std::unordered_set<std::string> seen;
+    for (const auto& b : banks_) {
+        for (const auto& s : b.index.all()) {
+            if (s.relativePath.empty()) continue;
+            if (seen.insert(s.relativePath).second)
+                paths.push_back(s.relativePath);
+        }
+    }
+    return paths;
+}
+
 // ===========================================================================
 // JSON — writer
 // ===========================================================================