Problem:
After closing and reopening Neovim, ]' and [' fail with E92: Buffer 0
not found for marks restored from ShaDa. Direct jumps like 'a work
because mark_get_local() rewrites fnum before returning, but ]' uses
getnextmark() which does not, leaving fnum = 0.
Solution:
Set .fnum = buf->b_fnum when restoring local marks from ShaDa.
0b96b3cd52