feat(cwd): support explicit chdir (:bcd/:tcd/…) in temp context

Problem:
- Explicit `:bcd` (etc.) persists from `nvim_buf_call()` but not from an
  autocmd handler targeting a hidden buf (`LspAttach`, `TermRequest`, …),
  which needs a `vim.schedule()` workaround.
- `vim._with()` is supposed to work as a "sandbox", discarding
  side-effects, but it leaks CWD changes: `:lcd` from a `win` context,
  any chdir from a visible-buffer context.

Solution:
- Explicit :cd/:tcd/:bcd during a temp context persists by default.
  - "Ambient" directory changes ('autochdir', existing win-local CWD,
    etc.) are still undone, as before.
- Add `kCtxKeepDirs`: snapshot/restore the target's full CWD state
  (w/b/tp-local, global, cwd). Used by `vim._with()` and `'inccommand'`,
  which must not leak state.

0 files changed, 0 insertions, 0 deletions