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/capture_realtime_shell.cpp

@@ -59,6 +59,7 @@
 #include "core/capture/capture_realtime.h" // RecordPhase machine, record-mode plan (pure)
 #include "core/capture/render_settings.h"  // realtimeRecordWindowEnd
 #include "shell/capture/capture_realtime_finalize.h"  // recordedFilePath, finalizeRecording
+#include "shell/persist/bank_root.h"
 
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_EnumProjects
@@ -84,20 +85,6 @@ namespace reasampler::capture {
 
 namespace {
 
-std::string normSlashes(std::string s) {
-    for (char& c : s) if (c == '\\') c = '/';
-    if (s.size() > 1 && s.back() == '/') s.pop_back();
-    return s;
-}
-
-// Reads the ACTIVE project's .rpp path (empty if unsaved). Only needed at begin()
-// time, when the record's project IS the active project.
-std::string readRppPath() {
-    std::vector<char> buf(4096, '\0');
-    EnumProjects(-1, buf.data(), static_cast<int>(buf.size()));
-    return std::string(buf.data());
-}
-
 // -1 if unresolved yet. Used by the flush wait to detect stability (size
 // unchanged across a tick) before moving the file — a take REAPER is still
 // flushing grows tick over tick.
@@ -290,20 +277,18 @@ RealtimeRecordBackend::begin(const CaptureRequest& request,
         return nullptr;
     }
 
-    // Saved-project gate (same as offline): the bank folder resolves against the
-    // .rpp parent. Prompt Save-As once when unsaved; refuse if still unsaved.
-    std::string rppPath = readRppPath();
-    if (rppPath.empty()) {
+    // Bank-root gate (same as offline): prompt Save-As once when there is no bank
+    // root; refuse if there still is none.
+    std::string projectDir = bankRootOf(proj);
+    if (projectDir.empty()) {
         Main_SaveProject(proj, true);
-        rppPath = readRppPath();
+        projectDir = bankRootOf(proj);
     }
-    if (rppPath.empty()) {
+    if (projectDir.empty()) {
         outFailure.status  = CaptureStatus::NoProject;
         outFailure.message = "Project must be saved before capture — nothing captured.";
         return nullptr;
     }
-    const std::string projectDir =
-        normSlashes(std::filesystem::path(rppPath).parent_path().string());
 
     RealtimeCaptureHandle st(new RealtimeCaptureState());
     st->proj_      = proj;