Fix the package fs seam: UTF-8 paths, GetUserFileName pickers, exclusive-create landing, rollback arm/disarm

Both pickers now ride GetUserFileName (mode 0/1); the "no save picker" premise was false.
Landing uses O_EXCL so the create is the existence check, not a TOCTOU pair.

src/shell/package/package_pickers.h

@@ -1,8 +1,7 @@
-// shell/package/package_pickers — the two package file pickers, and they are
-// deliberately asymmetric: import rides REAPER's own read picker
-// (GetUserFileNameForRead, both platforms); export goes native — Win32
-// GetSaveFileNameW / SWELL BrowseForSaveFile — because the always-present REAPER
-// surface offers no save picker. Do not symmetrize; the rationale is in the TU.
+// shell/package/package_pickers — the two package file pickers, both on REAPER's own
+// GetUserFileName (mode 1 = existing file, mode 0 = new file). No native/platform
+// split: the SDK's save mode is not optional on any build that can load this
+// extension. Paths in and out are UTF-8, per the REAPER API contract.
 
 #pragma once
 
@@ -10,12 +9,13 @@
 
 namespace reasampler {
 
-// REAPER's read picker. True with outAbsPath set iff the user chose a file.
+// True with outAbsPath set iff the user chose a file.
 bool pickPackageForImport(std::string& outAbsPath);
 
-// Native save picker, pre-filled with suggestedFileName (a bare name, e.g.
-// "MyBank.rsbank"). True with outAbsPath set iff the user chose a destination; the
-// dialog's own overwrite confirm has already run by then.
-bool pickPackageSavePath(const std::string& suggestedFileName, std::string& outAbsPath);
+// suggestedPath is a bare file name ("MyBank.rsbank") or a full path — a full one
+// also seeds the dialog's starting directory, which is how a caller keeps the picker
+// off REAPER's process working directory. True with outAbsPath set iff the user chose
+// a destination; the dialog's own overwrite confirm has already run by then.
+bool pickPackageSavePath(const std::string& suggestedPath, std::string& outAbsPath);
 
 }  // namespace reasampler