Problem:
Both handle lookups write into the same `Error` before either one is
checked, and `api_set_error()` unconditionally `xmalloc`s `err->msg`
without freeing what is already there. When both handles are invalid,
the message allocated for the window is silently overwritten by the one
for the buffer and leaked.

Solution:
Check the window before looking up the buffer. As a side effect the
reported error no longer names the buffer when the window was wrong too.

AI-assisted

Problem:
The default 'ruler' is implemented in C instead of the 'statusline' DSL.

Solution:
Replace the C implementation with a default 'rulerformat' expression.
This is a continuation of #1248 and #33036.

Advantages:
- configuration is more discoverable, the default being a useful example
- users and plugins can augment the default
- code reuse and less C code to maintain
- ui2: due to the use of an item group with `minwid`, it can expand
  instead of truncating when the content gets too long, which is
  particularly useful for locales with long translations of Top/Bot/All

Implementation details:
As is the case for 'statusline', when trying to set 'rulerformat' to an
empty string, the default expression is restored instead, mimicking how
previously the default C implementation would have been activated.
Just like before, `:set rulerformat=` and `:set rulerformat&` have the
same effect, and the ruler is disabled with `:set noruler`.

The default expression uses an item group with `%=`, unlike the fallback
in the previous default statusline `%-14.(%l,%c%V%) %P`, because the
total width and how it is configured is immediately clear without
documentation, it is a more useful pattern in general that works when
both sides have flexible width, and it also works for vim, which is
useful for configuration sharing/reuse.

A truncation marker `%<` is added at the end to mimic how at small
screen widths, the scroll percentage would disappear first, so that the
cursor position can remain fully visible.

BREAKING CHANGES:
- `&rulerformat` can no longer be set to an empty string
- ui2: the default ruler is no longer of fixed width, but can expand
- at very small screen widths (< 36 columns)
  - ui2: it will no longer try to shrink white-space before truncating
  - it truncates gradually from the right, whereas previously, the
    scroll percentage would disappear all at once
- l10n can no longer add a space after the comma between line and column
  (this was only done for one language: Ukrainian)

Problem: Traditionally, the ruler in the last line is one cell shorter
than in the statusline, leaving the last cell of the screen blank.
According to code comments, this is in order to prevent unwanted
scrolling on "some" (unspecified, but presumably ancient) terminals.
Berkeley vi is more specific in its `vs_modeline` function: dumb
terminals with hardware scroll, SunOS 4.1.1 and Ultrix 4.2 curses.
(n)curses still has a similar limitation in `(w)addstr`, but apparently
only for historical reasons.

Maintaining the different widths leads to awkward inconsistencies when
the ruler is configured with 'rulerformat', except for the special case
where it contains a top-level `%=`. Shifting the ruler in the last line
to the left would be a solution, but the empty cell at the end doesn't
seem to be relevant anymore.

Solution: extend the ruler in the last line all the way to the right
edge of the screen, just like in the statusline. The exact same amount
of place will be available to the rest of the UI as before.

BREAKING CHANGE:
- the default ruler width is now 18 cells
- the last cell of the screen is no longer empty

Closes #41076

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`.

Fixes a regression from #34486
Fix #40745

Problem: When a split window is created from the init script using nvim_open_win()
with enter=false, the current window focus is not preserved. The cursor
incorrectly moves into the newly created split during startup.

Solution: Save curwin before the iteration loop in create_windows(), and
restore it after the loop. also update edit_buffers() to use curwin instead
of firstwin.

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.