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/prune_reconcile.cpp

@@ -33,4 +33,23 @@ std::vector<std::string> pruneOrphans(const std::vector<std::string>& present,
     return orphans;
 }
 
+PruneReport buildPruneReport(
+    const std::vector<std::string>& orphans,
+    const std::unordered_map<std::string, std::uint64_t>& sizeByPath,
+    std::size_t displayCap) {
+    PruneReport report;
+    report.count = orphans.size();
+    for (const std::string& o : orphans) {
+        // Sum EVERY orphan's bytes (the exact reclaimable total), not just the displayed
+        // ones. A path with no stat'd size contributes 0 — never dropped, never negative.
+        const auto it = sizeByPath.find(o);
+        report.totalBytes += (it != sizeByPath.end()) ? it->second : 0;
+        // Display list is capped (0 = uncapped). count stays exact above regardless.
+        if (displayCap == 0 || report.orphans.size() < displayCap)
+            report.orphans.push_back(o);
+    }
+    report.truncated = report.orphans.size() < report.count;
+    return report;
+}
+
 } // namespace reasampler