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.h

@@ -1,7 +1,10 @@
 // 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.
+// extension. Paths in and out are UTF-8, matching this tree's established practice
+// for narrow strings crossing the REAPER API (see instrument_drop_win.cpp's
+// path.u8string() to TrackFX_SetPreset, or prune_fs.cpp's CP_UTF8 conversion) — the
+// SDK header itself never says "UTF-8".
 
 #pragma once