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.cpp

@@ -191,6 +191,13 @@ BankPaths deriveBankPaths(const std::string& projectDir,
     return p;
 }
 
+std::string bankRelativeForName(const std::string& fileName) {
+    if (fileName.empty()) return {};
+    // The SAME expression deriveBankPaths uses for relativePath, kept in one place so
+    // the two spellings can never drift (Phase R spelling-consistency invariant).
+    return std::string(kBankSubfolder) + "/" + fileName;
+}
+
 std::string resolveBankFile(const std::string& projectDir,
                             const std::string& relativePath) {
     // No default-location fallback (CLAUDE.md invariant): an empty project dir or