Route every extension bank-directory lookup through one bankRootOf seam

The pure bankRootFor(rppPath, unsavedRoot) and the shell bankRootOf(ReaProject*) replace eleven hand-rolled .rpp-parent derivations. No unsaved root is supplied yet, so behaviour and refusals are unchanged. Save-As relocation now reads the stored lastBankRoot_.

src/shell/persist/bank_root.cpp

@@ -0,0 +1,41 @@
+// bank_root.cpp — see bank_root.h. main.cpp owns the API pointers; this TU gets them
+// extern.
+
+#include "shell/persist/bank_root.h"
+
+#include <vector>
+
+#include "core/capture/capture_paths.h"  // bankRootFor
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_EnumProjects
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+namespace {
+
+// EnumProjects is the only SDK read of a project's .rpp path — GetProjectPathEx is the
+// recording path, GetProjectName the bare file name. Empty for a never-saved project.
+// The active project is read through idx -1 first, so the common case is one call.
+std::string rppPathOf(ReaProject* proj) {
+    std::vector<char> buf(4096, '\0');
+    ReaProject* const active = EnumProjects(-1, buf.data(), static_cast<int>(buf.size()));
+    if (!proj || proj == active) return std::string(buf.data());
+
+    for (int idx = 0;; ++idx) {
+        buf[0] = '\0';
+        ReaProject* const tab = EnumProjects(idx, buf.data(), static_cast<int>(buf.size()));
+        if (!tab) return {};  // not an open tab: no location to answer with
+        if (tab == proj) return std::string(buf.data());
+    }
+}
+
+}  // namespace
+
+std::string bankRootOf(ReaProject* proj) {
+    // No unsaved root is supplied: a never-saved project has no bank location.
+    return capture::bankRootFor(rppPathOf(proj), std::string{});
+}
+
+}  // namespace reasampler