feat(userregfunc): programmable user-defined registers with multibyte support

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.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index bf2bc077e0..fcb11fd281 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -96,6 +96,7 @@
 #include "nvim/move.h"
 #include "nvim/normal.h"
 #include "nvim/normal_defs.h"
+#include "nvim/ops.h"
 #include "nvim/option.h"
 #include "nvim/option_vars.h"
 #include "nvim/os/os_defs.h"
@@ -1184,8 +1185,8 @@ static void recording_mode(int hl_id)
   }
 
   msg_puts_hl(_("recording"), hl_id, false);
-  char s[4];
-  snprintf(s, ARRAY_SIZE(s), " @%c", reg_recording);
+  char s[4 + MB_MAXBYTES];
+  snprintf(s, ARRAY_SIZE(s), " @%s", reg_to_mb(reg_recording));
   msg_puts_hl(s, hl_id, false);
 }