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:
Literal path comparison ignored one trailing slash for every buffer name,
including URIs. Generic URI syntax does not make a non-empty path
equivalent to the same path with a trailing slash, so distinct URI
buffers collapsed into one.

Solution:
Require equal lengths when comparing URI buffer names, while retaining
trailing-separator normalization for filesystem paths.

AI-assisted

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:
Continue separators normalization, and try to keep it at the
nvim <-> external boundary, e.g., `fn.xxx`, `api.xxx`, `:xxx`

Solution:
some key changes
- normalize `$HOME-windows`
- normalize entry points of changing directory
  - `nvim_set_current_dir()`, `chdir()`
- normalize the named pipe
  - `--listen` arg, `--server` arg, `:restart`
  - `serverstart()`, `sockconnect()`, `serverstop()`
- make `expand()` respect 'shellslash' again
- clean up `did_set_shellslash`
- replace `forward_slash` with `TO_SLASH`
- remove obsolete `TMP_PATHSEPSTR`
- remove `slash_adjust` in `do_autocmd_dirchanged`?
- make `fnamemodify()` always return `/` (except when substituting
  separators via `:s`, `:gs` ?)

Note:
- these funcs still apply `slash_adjust` on return, as before:
  `:pwd`, `chdir()`, `exepath()`, `getcwd()`
- these funcs alwarys return `/`, unlike before the normalization PRs
  - `getcompletion()`, `finddir()`, `findfile()`

Also
- clean up code, comments, formatting and tests
- move `TO_SLASH` from `f_chdir` to `changedir_func`
- move `TO_SLASH` from `f_bufadd` to `buflist_new`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Problem:
`vim.filetype.match()` needs a cheap way to recognize directory buffers
without doing filesystem stat work.

Solution:
Ensure full buffer names for directories end in a trailing slash. Now
directory buffers can proceed through the normal 'filetype' path.

Note side-effects: session and ShaDa buffer-list restore behavior must
be compatible, so those + corresponding tests must be updated.

unify context-switching logic.

1. `prevwin` is now restored for all targets (was buf-only).
   - add a `nvim_win_call` test.
2. The buf-found "restore the shown buffer" dance no longer depends on
   the origin window: it runs even if the callback closed the origin.

Problem: changed_lines got a hardcoded 0, so the changelist entry
and '. mark always recorded column 0 instead of where the edit
actually happened.

Solution: pass start_col instead. changelist now tracks the real
column.

Problem:
Visual selection could end up in the wrong place after
nvim_buf_set_text or nvim_buf_set_lines. In some delete cases,
Visual.lnum was already clamped before the line shift happened, so the
adjustment got skipped.

Solution:
Split fix_cursor_cols into reusable fix_pos_col logic and reuse it
for Visual updates. Also adjust Visual.lnum before changed_lines so
the shift uses the original position before final clamping.