This patch introduces a new global option `userregfunc`, allowing users
to define custom behavior for registers not handled by Neovim
internally. This enables programmable registers using any Unicode
character — including multibyte characters.

- A new register slot `USER_REGISTER` is introduced. Any register not
  matching the standard set (`0-9a-zA-Z"+-*%#/:.=`, etc.) is routed
  through this system.

- When such a register is accessed, the function defined in
  `userregfunc` is called with three arguments:

    1. `{action}` (string): either `"yank"` or `"put"`
    2. `{register}` (string): UTF-8 character name of the register
    3. `{content}`:
       - If `action == "yank"`: a dictionary with these keys:
         - `lines` (list of strings): the yanked text
         - `type` (string): one of `"v"` (charwise), `"V"` (linewise), or `"b"` (blockwise)
         - `width` (number, optional): present if `type == "b"`
         - `additional_data` (dict, optional): user-extensible metadata
       - If `action == "put"`: this is always `v:null`

- The function may return either:
    - A **string** (used as a charwise register), or
    - A **dictionary** matching the structure above

- Internally, `read_userregister()` and `write_userregister()` convert
  between `yankreg_T` and typval dictionaries.

- Messages and internal logic fully support multibyte register names via
  UTF-8.

- A new `USER_REGISTER` slot is used for logical separation in the
  register table.

Included in this patch is an extensible Lua framework (`vim.userregs`)
for defining user register handlers in Lua. It provides per-register
handlers via `register_handler(registers, handler)`

The global function `_G.def_userreg_func` is registered as the default
implementation of `'userregfunc'`, enabling seamless integration with
the Lua framework.

- Register `λ` dynamically inserts the current date
- Register `&` reads and writes from a "global register" file under
  `stdpath("run")`
- Register `?` returns the result of a shell command
- Registers that auto-adjust based on filetype, cursor context, or
  Treesitter nodes

This change expands the register model into a programmable abstraction —
fully scriptable and extensible — without breaking compatibility.

Problem: vim.lsp.buf.hover allows a bold border size which hasn't been
defined

Solution: Define the bold border size for vim.lsp.buf.hover

Problem:
As checkhealth grows, it is increasingly hard to quickly glance through
the information.

Solution:
Show a summary of ok, warn, and error outputs per section.

Problem:  Using wrong window in ll_resize_stack()
          (after v9.1.1287)
Solution: Use "wp" instead of "curwin", even though they are always the
          same value.  Fix typos in documentation (zeertzjq).

closes: vim/vim#17080

https://github.com/vim/vim/commit/b71f1309a210bf8f61a24f4eda336de64c6f0a07

Problem:  quickfix and location-list stack is limited to 10 items
Solution: add the 'chistory' and 'lhistory' options to configure a
          larger quickfix/location list stack
          (64-bitman)

closes: vim/vim#16920

https://github.com/vim/vim/commit/88d41ab270a8390a43da97a903b1a4d76b89d330

Co-authored-by: 64-bitman <60551350+64-bitman@users.noreply.github.com>
Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>

Problem:  Help files not detected when 'iskeyword' includes ":".
Solution: Do not use \< and \> in the pattern (zeertzjq).

fixes: vim/vim#17069
closes: vim/vim#17071

https://github.com/vim/vim/commit/e370141bf41919642061ee2e78340dca84678712

Problem: In some cases, when treesitter is enabled, deleting a
line below virtualtext will not refresh all updated lines.
https://github.com/neovim/neovim/issues/33358

Solution: Revert a part of https://github.com/neovim/neovim/pull/31324
to ensure that the full range (with virtual lines) is refreshed.

Problem:
55e4301036bb938474fc9768c41e28df867d9286 changed the program name but not the function name.

Solution:
Fix the healthcheck.

Problem:
`client_is_stopped(…)` is an alias for `not get_client_by_id(…)`.
And it's not used anywhere in our codebase.

Solution:
Deprecate it.

Problem:
nvim_buf_del_extmark error if buffer is destroyed before timer stops

Solution:
check nvim_buf_is_valid.