Problem:
- nvim_buf_call() aborts (assert) when called from an expression evaluated
  for a window in another tabpage ('foldexpr', 'statusline'), because
  `curwin` is not in `curtab`.
- After it returns, `curwin` is wrong if it switched windows.
- `vim._with{win=…, keepcwd=true}` does not undo `:lcd` if `win` is in
  another tabpage.

Solution:
- Allow `curwin` outside `curtab`, and restore if it was outside
  `curtab` at `ctx_switch()`.
- Find the target window in any tabpage when restoring its dirs.

Problem:
`vim._with({ buf = 0, win = 0 }, f)` fails an assert in ctx_switch().
vim._with() allows `buf` with `win` when `win` shows `buf`, but
nlua_with() passes both, and ctx_switch() takes only one target.

Solution:
Ignore `buf` if `win` is given; the window implies its buffer.

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.

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem: No concise way to execute a callback with temporarily set
  working directory. This might be useful when sourcing nested files to
  allow them to assume that current working directory is their root
  directory.

Solution: Add `cwd` context to `vim._with().`

Problem: Using `buf=0`/`win=0` context in `vim._with` should be
  equivalent to using explicit buffer/window identifier respectively.

Solution: Explicitly adjust context in case of `buf=0` or `win=0`.