feat(mappings): vimwiki-parity default keymaps for Vim + Neovim

The Phase 19 keymap layer only covered the high-value subset and used
`g=`/`g-` instead of vimwiki's `=`/`-` for header levels. Users
reporting "mappings don't seem to be working" were either on the Vim
path (which registered zero keymaps) or hitting bindings that don't
exist (e.g. `<Tab>`, `<S-CR>`, `gll`, `gqq`).

Cross-referenced upstream vimwiki/ftplugin/vimwiki.vim and rewired
both `lua/nuwiki/keymaps.lua` and `ftplugin/vimwiki.vim` (Vim path)
to match its default surface. Mappings backed by commands that exist
on the server dispatch directly; §13.1-deferred mappings register
with the same lhs but stub out to a notification — so users get
parity *signalling*, not silence.

New on Neovim (and matched on Vim where possible):

Links group:
- `<S-CR>` / `<C-CR>` / `<C-S-CR>` — follow in split / vsplit / tab
- `<Tab>` / `<S-Tab>` — jump to next / prev `[[…]]` (pure Lua search)
- `+` (n + x) — :VimwikiNormalizeLink stub
- `<Leader>wc` (n + x) — :VimwikiColorize stub

Lists group:
- `<C-@>` (n + x) — terminal-alt for `<C-Space>`
- `gln` / `glp` increment + decrement (deferred backward; same fn for now)
- `glh` / `gll` / `gLh` / `gLl` — list level stubs
- `glr` / `gLr` — list renumber stubs
- `gl<Space>` / `gL<Space>` — checkbox remove stubs
- `o` / `O` — open below/above and continue the bullet (pure Lua)
- visual-mode variants of `<C-Space>`, `gln`, `glp`, `glx`

Header group (key group, now buffer-local):
- `=` / `-` — promote / demote (was `g=`/`g-`, matched to vimwiki)
- `]]` / `[[` — next / prev header (pure Lua)
- `]=` / `[=` — next / prev sibling header (pure Lua)
- `]u` / `[u` — parent header (pure Lua)

Table group: `gqq`, `gq1`, `gww`, `gw1`, `<A-Left>`, `<A-Right>` stubs.

Wiki prefix: `<Leader>w<Leader>m` (vimwiki's tomorrow), `<Leader>w<Leader>i` (rebuild diary index).

Config schema (`config.options.mappings`) updates to vimwiki's group
names — `links` / `lists` / `headers` / `table_editing` / `diary` /
`html_export` / `text_objects` / `wiki_prefix`. Old names
(`list_editing` / `header_nav`) are retired; rename only — keeps the
opt-out shape intact.

Vim path: same set ported to buffer-local VimL maps. `g:nuwiki_no_default_mappings`
opts out the whole layer for users who prefer their own bindings. Lua
side uses the Neovim 0.11+ `vim.keymap.set` API; both call straight
into the same `nuwiki#commands#…` autoload (Vim) / `nuwiki.commands`
(Lua) layer that drives the LSP.

Total 377 tests still pass.

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

lua/nuwiki/config.lua

@@ -11,15 +11,19 @@ M.defaults = {
   syntax         = 'vimwiki',
   log_level      = 'warn',
 
-  -- Phase 19: per-buffer glue. Each subgroup can be flipped off
-  -- independently to suppress the corresponding keymaps.
+  -- Phase 19: per-buffer glue. Each subgroup mirrors vimwiki's
+  -- `g:vimwiki_key_mappings` shape and can be flipped off
+  -- independently to suppress that group of keymaps.
   mappings = {
     enabled       = true,
-    list_editing  = true,
-    header_nav    = true,
-    diary         = true,
-    html_export   = true,
-    text_objects  = true,
+    wiki_prefix   = true,  -- <Leader>w*
+    links         = true,  -- <CR>, <S-CR>, <Tab>, <BS>, +, …
+    lists         = true,  -- <C-Space>, gln/glp/glx, gl<Space>, o/O, …
+    headers       = true,  -- =, -, ]], [[, ]=, [=, ]u, [u
+    table_editing = true,  -- gqq, gq1, gww, gw1, <A-Left>, <A-Right>
+    diary         = true,  -- <C-Down>, <C-Up>
+    html_export   = true,  -- <Leader>wh, <Leader>whh
+    text_objects  = true,  -- ah, ih, …
   },
 
   -- Phase 19: folding. `'lsp'` uses the server's `foldingRange`