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/package/import_bank.cpp

@@ -1,5 +1,5 @@
 // import_bank.cpp — see import_bank.h for the contract. The REAPER-facing half of the
-// import: the project directory, the minted bank id, the birth records, and the one
+// import: the bank root, the minted bank id, the birth records, and the one
 // undo-batched persist. Every decision it makes is in import_landing / import_plan.
 //
 // main.cpp owns the API pointers; this TU gets them extern. DAW-verified, not unit tested.
@@ -11,12 +11,11 @@
 #include <string>
 #include <vector>
 
-#include "core/capture/capture_paths.h"  // projectDirOfRpp
 #include "shell/bank_ops/bank_ops.h"     // persistBankOp — one bank op is one Ctrl-Z
+#include "shell/persist/bank_root.h"
 #include "shell/persist/session.h"
 
 #define REAPERAPI_MINIMAL
-#define REAPERAPI_WANT_EnumProjects
 #define REAPERAPI_WANT_genGuid
 #define REAPERAPI_WANT_guidToString
 #include "reaper_plugin_functions.h"
@@ -25,12 +24,6 @@ namespace reasampler {
 
 namespace {
 
-std::string activeProjectDir() {
-    std::vector<char> buf(4096, '\0');
-    EnumProjects(-1, buf.data(), static_cast<int>(buf.size()));
-    return capture::projectDirOfRpp(std::string(buf.data()));
-}
-
 // The model mints no ids (it stays pure and deterministic), so the shell does — the
 // same GUID pair bankOpCreate uses.
 std::string mintBankId() {
@@ -62,7 +55,7 @@ ImportBankResult importBankPackage(ReaSamplerSession& session,
         std::to_string(static_cast<std::int64_t>(std::time(nullptr)));
 
     LandedFileJournal journal;
-    const ImportLanding landing = landPackage(packageAbsPath, activeProjectDir(),
+    const ImportLanding landing = landPackage(packageAbsPath, bankRootOf(nullptr),
                                               session.book(), uniqueTag, journal);
     out.outcome = landing.outcome;
     out.header = landing.header;