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/core/capture/capture_paths.cpp

@@ -106,6 +106,12 @@ std::string projectDirOfRpp(const std::string& rppPath) {
     return normalizeSlashes(dir);
 }
 
+std::string bankRootFor(const std::string& rppPath, const std::string& unsavedRoot) {
+    const std::string projectDir = projectDirOfRpp(rppPath);
+    if (!projectDir.empty()) return projectDir;
+    return normalizeSlashes(unsavedRoot);
+}
+
 BankRelocation deriveRelocationPlan(const std::string& oldProjectDir,
                                     const std::string& newProjectDir) {
     BankRelocation r;