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.
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