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/capture/insert.cpp

@@ -32,12 +32,12 @@
 #include "core/model/bank_model.h"
 #include "shell/panel/panel_bank_ops.h"  // bankPanelSelectedSampleIds / SourceBankId
 #include "core/capture/capture_paths.h"
+#include "shell/persist/bank_root.h"
 #include "shell/persist/session.h"
 #include "shell/undo/undo_block.h"
 
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_CountSelectedTracks
-#define REAPERAPI_WANT_EnumProjects
 #define REAPERAPI_WANT_GetCursorPosition
 #define REAPERAPI_WANT_GetSelectedTrack
 #define REAPERAPI_WANT_InsertMedia
@@ -50,7 +50,6 @@
 namespace reasampler {
 
 using capture::computeInsertMode;
-using capture::normalizeSlashes;
 using capture::resolveBankFile;
 using capture::TempoConform;
 
@@ -58,18 +57,6 @@ namespace {
 
 namespace fs = std::filesystem;
 
-// Mirrors bank_panel/capture/persist's own derivation; the bank index stores
-// relative paths, so resolving a file needs the current .rpp dir. A shared "current
-// project dir" helper would be a clean small refactor now that a fourth consumer
-// exists (also noted in panel_bank_ops.cpp) — out of scope here.
-std::string currentProjectDir() {
-    std::vector<char> buf(4096, '\0');
-    EnumProjects(-1, buf.data(), static_cast<int>(buf.size()));
-    std::string rpp(buf.data());
-    if (rpp.empty()) return {};  // unsaved project: no resolvable bank
-    return normalizeSlashes(fs::path(rpp).parent_path().string());
-}
-
 // Snapshot of the currently-selected track set (master is skipped, matching
 // CountSelectedTracks/GetSelectedTrack), in selection order, for restore later.
 std::vector<MediaTrack*> snapshotSelectedTracks() {
@@ -111,9 +98,9 @@ InsertResult runInsert(ReaSamplerSession* session, const InsertRequest& request)
     if (ids.empty()) { result.status = InsertStatus::NoSelection; return result; }
     const std::string& id = ids.front();  // focused / first selected — single sample
 
-    // WHERE: an unsaved project has no resolvable bank dir; no-op rather than
+    // WHERE: no bank root means no resolvable bank file; no-op rather than
     // resolving against CWD.
-    const std::string projectDir = currentProjectDir();
+    const std::string projectDir = bankRootOf(nullptr);
     if (projectDir.empty()) { result.status = InsertStatus::NoProject; return result; }
 
     // Resolve against the bank the selection came from — it may be the pool or a