fix(shada): set correct buffer number for local marks on read #39712

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.

src/nvim/shada.c

@@ -1180,7 +1180,7 @@ static void shada_read(FileDescriptor *const sd_reader, const int flags)
       }
       const fmark_T fm = (fmark_T) {
         .mark = cur_entry.data.filemark.mark,
-        .fnum = 0,
+        .fnum = buf->b_fnum,
         .timestamp = cur_entry.timestamp,
         .view = INIT_FMARKV,
         .additional_data = cur_entry.additional_data,

test/functional/shada/marks_spec.lua

@@ -189,6 +189,17 @@ describe('ShaDa support code', function()
     eq(2, nvim_current_line())
   end)
 
+  it("is able to dump, read back local mark and use `]'`", function()
+    nvim_command('edit ' .. testfilename)
+    nvim_command('2')
+    nvim_command('mark a')
+    expect_exit(nvim_command, 'qall')
+    reset({ args = { testfilename } })
+    eq(1, nvim_current_line())
+    nvim_command("normal! ]'")
+    eq(2, nvim_current_line())
+  end)
+
   it('is able to dump and read back mark " from a closed tab', function()
     nvim_command('edit ' .. testfilename)
     nvim_command('tabedit ' .. testfilename_2)