Close package fs review findings: readRange bounds, picker ext, non-ASCII tests

Cap readRange's allocation and reject size_t overflow instead of truncating;
re-append .rsbank when the export picker omits it; add cafe coverage for
writeFileExclusive and writeLandedFile; loop write() on EINTR.

src/shell/package/package_pickers.cpp

@@ -5,6 +5,9 @@
 
 #include "shell/package/package_pickers.h"
 
+#include <algorithm>
+#include <cctype>
+
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_GetUserFileName
 #include "reaper_plugin_functions.h"
@@ -30,6 +33,14 @@ bool runPicker(int mode, const char* caption, const char* initial,
     return !outAbsPath.empty();
 }
 
+bool hasCaseInsensitiveSuffix(const std::string& path, const std::string& suffix) {
+    if (path.size() < suffix.size()) return false;
+    return std::equal(suffix.rbegin(), suffix.rend(), path.rbegin(),
+                      [](unsigned char a, unsigned char b) {
+                          return std::tolower(a) == std::tolower(b);
+                      });
+}
+
 }  // namespace
 
 bool pickPackageForImport(std::string& outAbsPath) {
@@ -39,7 +50,14 @@ bool pickPackageForImport(std::string& outAbsPath) {
 bool pickPackageSavePath(const std::string& suggestedPath, std::string& outAbsPath) {
     // [verify — DAW] GetUserFileName takes no owner window, so the dialog's parenting
     // is REAPER's to do; the previous Win32 path passed GetMainHwnd() explicitly.
-    return runPicker(0, "Export bank package", suggestedPath.c_str(), outAbsPath);
+    if (!runPicker(0, "Export bank package", suggestedPath.c_str(), outAbsPath)) {
+        return false;
+    }
+    // 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";
+    return true;
 }
 
 }  // namespace reasampler