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.h

@@ -220,6 +220,17 @@ public:
     bool hashReferencedElsewhere(const std::string& hash,
                                  const std::string& exceptBankId) const;
 
+    // Every project-relative file path referenced by ANY bank in the book, pool
+    // included — the union across the whole book (Phase R, prune). This is the
+    // safety-critical referenced-set the prune core subtracts: a file referenced by
+    // any bank (INCLUDING via a copy into a second bank) appears here, so prune never
+    // reclaims it. Paths are returned VERBATIM (Sample.relativePath, exact strings —
+    // no normalization), first-seen order across banks in ordinal order then sample
+    // insertion order, and DE-DUPLICATED (one file referenced by N banks appears
+    // once). An empty relativePath is skipped (it references no file). Additive
+    // read-only query; adds no mutation and no coupling to Phase R.
+    std::vector<std::string> referencedPaths() const;
+
     // -- Query ---------------------------------------------------------------
 
     // The bank with `id`, or nullptr. Pointer invalidated by any mutating call.