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/persist.h

@@ -23,6 +23,7 @@
 #include "bank_book.h"
 #include "bank_model.h"
 #include "owned_manifest.h"
+#include "prune_reconcile.h"
 #include "tail_control.h"
 #include "view_mode_model.h"
 
@@ -179,6 +180,21 @@ public:
     // no dangling no-effect undo entry is opened on an unsaved project.
     bool saveToActiveProject();
 
+    // Compute the Phase R prune dry-run for the ACTIVE project (Wave 2 — REPORT ONLY,
+    // deletes nothing). Enumerates the resolved CURRENT bank folder (the SAME M4 project-
+    // relative machinery the index/persist use — never a stale absolute path, so it is
+    // correct across a Save-As relocation), spells every enumerated entry with the index's
+    // own convention (bankRelativeForName — byte-identical to the capture path's spelling),
+    // and feeds the R1 pure core with (present, book().referencedPaths(), owned().paths()).
+    // Returns the orphan count + reclaimable bytes + the (possibly display-truncated) file
+    // list. The decision stays in the pure core — this method only enumerates, resolves,
+    // and stats. READ-ONLY across the whole persist seam: it writes NO ext-state, calls no
+    // save / MarkProjectDirty, and mutates neither the book, the manifest, nor any file.
+    //
+    // Yields an empty report (count 0) when there is no active/saved project or no bank
+    // folder on disk yet — an unsaved or never-captured project has nothing to reclaim.
+    PruneReport pruneDryRun() const;
+
     // Poll the active project. Detects a project load (active project changed)
     // and a Save-As (active project's .rpp path changed) and reacts accordingly.
     // Intended to be driven by REAPER's "timer" register. Idempotent per tick.