Problem:
POSIX-compatible Ex-mode requires special-cases all over the codebase to
match various quirks that don't actually matter to users.
- The main utility of *interactive* Ex-mode is its REPL behavior, and
  that can be achieved with `cmdwin`, which also gains extra UX
  benefits.
- The main utility of *non-interactive* `nvim -es` is for shell
  scripting, where Ex-mode quirks are mostly unhelpful (e.g. the
  "Entering Ex mode" message).

Solution:
- Reimplement *interactive* Ex-mode as a "persistent, insert-mode
  cmdwin" in Lua.
  - "nvim -e/-E" is simply an alias to "gQ".
- Reframe *non-interactive* Ex-mode (`nvim -es`) as "script mode".
  - Drop POSIX Ex-mode quirks.

Improvements:
- "nvim -V1 -es" output ends with a final newline!
- "nvim -V1 -es" no longer shows the "Entering Ex mode" msg. (This was
  pointless noise, unwanted for scripting purposes.)
- stdin is no longer typeahead. Scripts (":lua io.read()") can read
  stdin as data.
- Empty line is a no-op: a stray blank line no longer moves the cursor
  (deviates from POSIX ex "+1"), no longer exits 1 at EOF (E501).

Preserved behavior:
- cursor starts at "$"
- mode()=="cv" (for non-interactive)
- multiline commands (:append/:function/heredoc pull continuation lines)
- bare-range print
- :print=>stdout
- -V1=>stderr
- CRLF input
- continue-after-error and exit codes

Dropped (regressed) POSIX behavior (non-interactive):
- Event loop only ticks while/between commands, not while blocked
  waiting for a stdin line.
- ":g/pat/visual...Q"
- input()/getchar()/":s/x/y/c" no longer consume stdin lines as
  answers: Nvim stops at end-of-input, skipping the rest of the script,
  exit 0. Use ":lua io.read()" instead.
  - If users care about this they should use interactive Ex-mode (`gQ`).
- ":@r" stops at end of the register instead of continuing to read
  cmdline input from stdin.

Move vim.wait into runtime/lua/vim/_core/editor.lua and replace
the C entrypoint with narrow vim._core helpers for polling, UI
flushing, and interrupt checks.

Keep the existing interval semantics by retaining the dummy timer that
wakes the loop while it is otherwise idle.

Update the docs to describe the success return values correctly, and
adjust the test expectation for the new vim.validate() callback error.

AI-assisted: Codex

Problem:
- Not obvious which _meta/ are generated and which should be edited
  manually.
- The require guard (`error('Cannot require a meta file')`) is not
  consistently present in all meta files.

Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.

Use the wording from the wait() Vimscript function.

Problem:
Using vim.defer_fn() just before Nvim exit leaks luv handles.

Solution:
Make vim.schedule() return an error message if scheduling failed.
Make vim.defer_fn() close timer if vim.schedule() failed.

Problem:  Fast context for msg_show event inhibits vim.ui_attach from
          displaying a stream of messages from a single command.

Solution: Remove fast context from msg_show events emitted as a result
          of explicit API/command calls. The fast context was originally
          introduced to prevent issues with internal messages.

fix https://github.com/neovim/neovim/issues/36858

Problem:
After bc0635a9fc9c15a2423d4eb35f627d127d00fa46 `vim.wait()` rejects floats
and NaN values.

Solution:
Restore the prior behavior, while still supporting `math.huge`. Update
tests to cover float case.

Close #35926
Close #35818

Co-authored-by: skewb1k <skewb1kunix@gmail.com>
Co-authored-by: glepnir <glephunter@gmail.com>