Close bank-export review findings: name-cap underflow, double overwrite prompt, test scope

Clamps insertSuffix's underflow, floors uniqueEntryName's validity guard, suppresses
the redundant overwrite confirm via a picker out-param, adds a PayloadBuffer
high-water mark, and corrects stale CLAUDE.md/CMake claims.

src/shell/package/package_pickers.cpp

@@ -47,7 +47,8 @@ bool pickPackageForImport(std::string& outAbsPath) {
     return runPicker(1, "Import bank package", "", outAbsPath);
 }
 
-bool pickPackageSavePath(const std::string& suggestedPath, std::string& outAbsPath) {
+bool pickPackageSavePath(const std::string& suggestedPath, std::string& outAbsPath,
+                         bool* outAppended) {
     // [verify — DAW] GetUserFileName takes no owner window, so the dialog's parenting
     // is REAPER's to do; the previous Win32 path passed GetMainHwnd() explicitly.
     if (!runPicker(0, "Export bank package", suggestedPath.c_str(), outAbsPath)) {
@@ -56,7 +57,9 @@ bool pickPackageSavePath(const std::string& suggestedPath, std::string& outAbsPa
     // GetUserFileName has no lpstrDefExt equivalent (the old Win32 picker's
     // ofn.lpstrDefExt = L"rsbank"); whether mode 0 appends one itself from
     // kExtList is [verify — DAW], so append it ourselves whenever it's missing.
-    if (!hasCaseInsensitiveSuffix(outAbsPath, ".rsbank")) outAbsPath += ".rsbank";
+    const bool appended = !hasCaseInsensitiveSuffix(outAbsPath, ".rsbank");
+    if (appended) outAbsPath += ".rsbank";
+    if (outAppended) *outAppended = appended;
     return true;
 }