feat(prune): R2 dry-run prune shell + persist wiring, report-only

Add ReaSamplerSession::pruneDryRun enumerating the resolved current bank
folder, feeding the R1 core, and returning a PruneReport (count/bytes/list).
New pure bankRelativeForName + buildPruneReport keep spelling and tally
testable. Register forever-stable BANK_PRUNE_FOLDER action, report-only. No deletion.

src/capture_paths.h

@@ -92,6 +92,18 @@ BankPaths deriveBankPaths(const std::string& projectDir,
                           const std::string& baseName,
                           const std::string& uniqueTag);
 
+// The project-relative index spelling for a bank file KNOWN ONLY by its file name —
+// the forward derivation the Phase R prune shell uses to spell an ENUMERATED folder
+// entry the SAME way deriveBankPaths spelled it at capture time. By construction it
+// is the identical expression deriveBankPaths().relativePath uses (kBankSubfolder +
+// "/" + fileName), so a file the capture path created and a directory listing of that
+// same file resolve to the byte-identical relative string — the safety-critical
+// spelling-consistency the prune core's exact-string match depends on (a divergence
+// here could make a referenced file look like an orphan). fileName is a bare entry
+// name (no directory component); the caller supplies forward-slash-free names from the
+// folder enumeration. Empty in -> empty out.
+std::string bankRelativeForName(const std::string& fileName);
+
 // --- Persist-side path arithmetic (M4) --------------------------------------
 //
 // The index stores relative paths only; on project load the persist shell must