feat(keymaps): configurable map_prefix for the wiki command family (P2)

Mirror vimwiki's g:vimwiki_map_prefix. The whole <Leader>w* family was
hardcoded across the map layer; it is now built from a configurable prefix:

- Neovim: `map_prefix` setup() option (default '<Leader>w'), read by
  lua/nuwiki/keymaps.lua (buffer-local) and lua/nuwiki/init.lua (global
  entry points).
- Vim: g:nuwiki_map_prefix, applied via :exe in plugin/nuwiki.vim (global)
  and ftplugin/vimwiki.vim (buffer-local).

Setting a custom prefix relocates <prefix>{w,t,s,i,n,d,r,c,h,hh,ha} and
<prefix><Leader>{w,y,t,m,i}, leaving nothing under the old <Leader>w*.
Non-prefix maps (<CR>, gl*, headers, …) are untouched.

Tests: new test-keymaps-vim-prefix.vim harness (21 cases, wired into
test-keymaps-vim.sh) + map_prefix.relocates_wiki_family in test-keymaps.lua.
Docs: README, doc/nuwiki.txt, lua config comment, vimwiki-gap.md ticked.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

doc/nuwiki.txt

@@ -103,6 +103,7 @@ default): >
     file_extension = '.wiki',
     syntax         = 'vimwiki',
     log_level      = 'warn',
+    map_prefix     = '<Leader>w',
   })
 
 <For multiple wikis or per-wiki tuning, use the `wikis` list: >
@@ -132,6 +133,7 @@ plugin: >
   let g:nuwiki_wiki_root           = '~/vimwiki'
   let g:nuwiki_file_extension      = '.wiki'
   let g:nuwiki_log_level           = 'warn'
+  let g:nuwiki_map_prefix          = '<Leader>w'
 
 <Top-level options ~
 
@@ -152,6 +154,12 @@ plugin: >
 `log_level`        warn
     `error` | `warn` | `info` | `debug`. Forwarded to the language server.
 
+                                                       *nuwiki-config-map-prefix*
+`map_prefix`       '<Leader>w'
+    Prefix for the wiki command family (`<prefix>w`, `<prefix>t`,
+    `<prefix><Leader>w`, …). Mirrors vimwiki's `g:vimwiki_map_prefix`. Vim
+    users set |g:nuwiki_map_prefix| instead.
+
                                                        *nuwiki-config-folding*
 `folding`          'lsp'
     `lsp` uses the server's `foldingRange` provider (Neovim 0.11+ wires
@@ -253,6 +261,14 @@ Vim-specific globals ~
         `html_export`     `<Leader>wh`, `<Leader>whh`, `<Leader>wha`
         `text_objects`    `ah`, `ih`, `al`, `il`, `a\`, `i\`, `ac`, `ic`
 
+                                                        *g:nuwiki_map_prefix*
+`g:nuwiki_map_prefix`          `'<Leader>w'`
+    Prefix for the wiki command family — both the global entry-point maps
+    (`<prefix>w`, `<prefix>t`, `<prefix>s`, `<prefix>i`, `<prefix><Leader>{w,y,
+    t,m,i}`) and the buffer-local ones (`<prefix>{n,d,r,c,h,hh,ha}`). Mirrors
+    vimwiki's `g:vimwiki_map_prefix`. Set it before the plugin loads. Neovim
+    users set the `map_prefix` option in `setup()` instead.
+
                                                           *g:nuwiki_no_folding*
 `g:nuwiki_no_folding`          0
     When 1, the Vim path skips `foldexpr` / `foldmethod` setup.