fix(cwd): keep buffer-local dir when re-editing #41215

Problem:
Re-editing a buffer (`:edit!`, re-reading a dir.lua buffer, etc.) drops
its `:bcd` directory, so the CWD falls back to the global one. Whereas
other buffer-local state (`b:` vars, local options) survives a reload.

Solution:
Don't clear buf dir in `buf_freeall()`; `do_ecmd()` calls that when
reloading/re-editing. `free_buffer_stuff()` still clears them when
a buffer is freed or reused for another file.

test/functional/ex_cmds/cd_spec.lua

@@ -436,6 +436,12 @@ for _, cmd in ipairs { 'bcd', 'bchdir' } do
       command('edit ' .. tmpfile .. '2')
       eq(startdir, cwd())
       eq(0, blwd())
+
+      -- But re-editing the same buffer keeps it, like any other buffer-local state. #41213
+      command(('%s %s'):format(cmd, directories.buffer))
+      command('let b:kept = 1')
+      command('edit!')
+      eq({ 1, bufdir, 1 }, { blwd(), cwd(), n.eval('get(b:, "kept", 0)') })
     end)
 
     it('is not cleared or overridden by :lcd/:tcd/:cd', function()