fix(shada): preserve marks after delete across sessions (#35795)

Problem: Marks are lost after `:bdelete` because `ignore_buf()` filters
out unlisted buffers during shada mark processing.

Solution: Revert to original buffer checks for mark operations, avoiding
`!buf->b_p_bl` condition that incorrectly ignores deleted buffers.

src/nvim/shada.c

@@ -2443,7 +2443,8 @@ static ShaDaWriteResult shada_write(FileDescriptor *const sd_writer,
         fname = fm.fname;
       } else {
         const buf_T *const buf = buflist_findnr(fm.fmark.fnum);
-        if (ignore_buf(buf, &removable_bufs)) {
+        if (buf == NULL || buf->b_ffname == NULL
+            || set_has(ptr_t, &removable_bufs, (ptr_t)buf)) {
           continue;
         }
         fname = buf->b_ffname;

test/functional/shada/marks_spec.lua

@@ -418,4 +418,30 @@ describe('ShaDa support code', function()
     -- Make sure that uppercase marks aren't deleted.
     nvim_command('normal! `A')
   end)
+
+  it('preserves marks after :bdelete #35770', function()
+    nvim_command('edit foo')
+    nvim_command('mark X')
+    nvim_command('bdelete')
+    nvim_command('wshada')
+
+    reset()
+    local mX = api.nvim_get_mark('X', {})
+    t.matches(vim.pesc('foo'), mX[4])
+
+    nvim_command('delmarks X')
+    nvim_command('edit foo')
+    nvim_command('mark X')
+    nvim_command('edit bar')
+    nvim_command('mark Y')
+    nvim_command('bufdo bdelete')
+    nvim_command('edit foo')
+    nvim_command('wshada')
+
+    reset()
+    mX = api.nvim_get_mark('X', {})
+    local mY = api.nvim_get_mark('Y', {})
+    t.matches(vim.pesc('foo'), mX[4])
+    t.matches(vim.pesc('bar'), mY[4])
+  end)
 end)