fix(keymap): C-Space toggle fires across all terminal byte spellings

User reported Ctrl+Space didn't toggle list items. Three things
contributed:

1. **Terminal byte ambiguity.** Different terminals encode Ctrl+Space
   differently — GUI / kitty protocol sends `<C-Space>`, xterm and
   most legacy terms send NUL which Vim sees as `<C-@>` (or, in some
   Neovim builds, `<Nul>` in keymap.lhs). The previous keymap
   registered only `<C-Space>` + `<C-@>`. Added the `<Nul>` alias too
   (both Lua and VimL paths) so the binding fires regardless of what
   byte the user's terminal produces.

2. **Deprecated `vim.lsp.util.make_position_params`.** Neovim 0.12
   requires `position_encoding` explicitly. The old no-arg call now
   prints a deprecation warning, and in stricter setups returns nil
   — which made the command's `arguments` malformed and the
   `executeCommand` request silently fail. `position_params()` now
   pulls `offset_encoding` from the resolved client and passes both
   args, with a `pcall`-guarded fallback to the no-arg form for
   Neovim < 0.10.

3. **Deprecated `client.request(...)` dot-call.** Same Neovim 0.12
   change — `Client:request()` is the method form. Use the colon
   call now; both arities are preserved for back-compat.

Verified end-to-end: feeding all three of `<C-Space>` / `<C-@>` /
`<Nul>` via `nvim_feedkeys` toggles the checkbox; `:messages` no
longer carries the deprecation warnings. 381 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -146,6 +146,8 @@ if !has('nvim')
     xnoremap <silent><buffer> <C-Space>  :<C-u>call nuwiki#commands#toggle_list_item()<CR>
     nnoremap <silent><buffer> <C-@>      :call nuwiki#commands#toggle_list_item()<CR>
     xnoremap <silent><buffer> <C-@>      :<C-u>call nuwiki#commands#toggle_list_item()<CR>
+    nnoremap <silent><buffer> <Nul>      :call nuwiki#commands#toggle_list_item()<CR>
+    xnoremap <silent><buffer> <Nul>      :<C-u>call nuwiki#commands#toggle_list_item()<CR>
     nnoremap <silent><buffer> gnt        :call nuwiki#commands#next_task()<CR>
     nnoremap <silent><buffer> gln        :call nuwiki#commands#cycle_list_item()<CR>
     xnoremap <silent><buffer> gln        :<C-u>call nuwiki#commands#cycle_list_item()<CR>