Two more digit-stem-only callsites I missed in the saved-dot fix
sweep. _set_library_info and _show_library_post both did
'if not stem.isdigit(): return' before consulting library_meta or
building the toolbar Post. Templated files (post-template-refactor
saves like 12345_hatsune_miku.jpg) bailed out silently — clicking
one in the Library tab left the info panel showing the previous
selection's data and the toolbar actions did nothing.
Extracted a small helper _post_id_from_library_path that resolves
either layout: look up library_meta.filename first (templated),
fall back to int(stem) for legacy digit-stem files. Both call sites
go through the helper now.
Same pattern as the find_library_files / _is_post_in_library
fixes from the earlier saved-dot bug. With this commit there are
no remaining "is templated file in the library?" callsites that
fall back to digit-stem matching alone — every check is
format-agnostic via the DB.