Problem:
`nvim_set_option_value('fo', {'c','r','o'}, {operation='remove'})` does
nothing on "jcqrol": the items are joined into one `:set-=` value, which
only matches a contiguous substring. Removing map keys, e.g. {'eol','space'}
from 'listchars', never matches. `vim.opt:remove{…}` inherits this.

(Regression from f34ee3da80e8; 429338e74bf4 fixed the case where the
flags are adjacent and in order.)

Solution:
Remove each item separately.

Problem: No way to check if cascade is in action.

Solution: Add `v:cascading` variable.

Problem:
Only the stdio channel can "own" a server (self-exit on close). After
:restart the TUI reconnects over a socket, so the restarted server does
not have "self-exit" behavior when the channel closes (e.g. by UI
disconnect, host-terminal close, etc.), which leaves a dangling server.

Solution:
- Introduce `nvim_chan_set`. (`nvim_set_client_info` doesn't fit:
  exposing it to Lua would require a `channel` param, which is not
  backwards-compatible, plus it's awkward because it doesn't merge info.)
- Broadcast per-channel properties in the "restart" UI event as an
  opaque dict (a potentially useful pattern for other UI-events in the
  future).
  - UIs are expected to restore the channel properties on restart.
- `nvim_get_chan_info()` reports "detach".
- Remove `nvim__chan_set_detach()`.

Problem: End-of-line virtual text is drawn directly after a displayed
listchars eol character.

Solution: Preserve the normal one-cell separator after the eol
character.

AI-assisted

Problem:
`nvim_mcursor` is not fully baked, and not strongly motivated,
currently. The main programmatic interface for cursors is the
"nvim.multicursor" extmarks namespace.

Solution:
Drop `nvim_mcursor`. Add an internal hook so that multicursor can react
when extmarks are added to the "nvim.multicursor" namespace.

TODO:
If extmarks/namespaces are clumsy to work with, we should _fix that_
rather than adding wrappers.

Problem: Command inputs and parsed modifiers have different optional
fields, and completion window IDs are annotated as floating-point
values.

Solution: Separate modifier input and return types, correct integer IDs,
and update the metadata generators and their output.

AI-assisted

Problem:

nvim_set_option_value() with dry_run=true accepts invalid values such as
foldmethod='nonsense'. Validation is mixed with side effects in the
did_set_* callbacks, so dry runs skip it.

Solution:

Split value validation out of the did_set_* callbacks into separate
validation_cb callbacks. Use these checks for both normal assignments
and nvim_set_option_value() dry runs. Keep side effects in did_set_*.

Replace direct foldenable/foldlevel assignments in C folding commands
with setters. This gives C commands the same OptionSet and
synchronization behavior as Lua assignments.

Use normal setters for 'tabstop' and 'vartabstop' in :retab so C code
indentation and folds based on indentation use the new tab widths.
Direct assignments could leave them using the old widths.

:retab temporarily turns off 'list' to calculate tab widths. Previously,
:retab -1 reported an error and returned without restoring 'list'. Check
the argument before turning 'list' off, so an invalid argument leaves
the option unchanged.

AI-assisted

Problem: Autocmd callbacks currently have to rely on curwin, which may
already have changed by the time they run. This is especially noticeable
for WinNew, WinClosed, WinResized and WinScrolled.

Solution: Track the window in the autocmd context and expose it as ev.win.
Pass it explicitly for the window events that need it.

Other (squashed) commits:

fix(tui): emit ui_send output atomically with the frame

Problem:
tui_ui_send() writes directly to the TTY, bypassing the output buffer.
Sequences sent via nvim_ui_send() (e.g. kitty multiple-cursors, or
visual-dot-repeat) always arrive in a separate TTY write from the frame
they were computed for. This manifests as "tearing", or e.g. in the case
of multicursor the terminal renders text with stale cursor overlays.

Solution:
- tui_ui_send(): while a frame is being assembled (pending invalid
  regions or buffered output), buffer instead of writing directly.
  - Out-of-frame sends (tty queries, clear-on-disable) still write
    immediately.
- mcursor.lua: emit the terminal-cursor update at the end of the redraw
  cycle (`on_end`, when screen positions are final) instead of
  vim.schedule().

Co-authored-by: darkdi <rantovov5@gmail.com>
Co-authored-by: Qiaoxi Guo <28090444+breadtitor@users.noreply.github.com>
Co-authored-by: zaveshaa <zaveshaa@gmail.com>