phase 19: editor glue v2

Server-side:
- New `folding.rs` module with `folding_ranges(ast, total_lines)`.
  Emits one fold per top-level heading (start → line before the
  next same-or-higher-level heading, or EOF) plus one per top-level
  list block. Nested headings fold inside their parent thanks to the
  level-aware end-line computation; sublists fold implicitly via
  their parent item's source span. `FoldingRangeKind::Region` is set
  on every fold so collapsing UIs render them as section folds.
- `Backend::folding_range` handler wires the module into
  `textDocument/foldingRange`; `ServerCapabilities.folding_range_provider`
  advertises it.
- `folding::line_count` exposed for the handler and tests; treats a
  trailing newline as a virtual empty line (the line LSP positions
  use for EOF).

Editor glue (Neovim primary, Vim minimal):
- `lua/nuwiki/commands.lua` — async `workspace/executeCommand`
  wrappers for every server command shipped through Phase 18. The
  open-* family handles `{ uri }` responses by opening the file
  (with `tab` / `split` variants). `check_links` and `find_orphans`
  hand results to `setqflist` + `:copen` for quickfix-style review.
  §13.1-deferred commands (`list.changeSymbol`, table rewriters,
  `link.pasteWikilink/pasteUrl`, `colorize`) stub out with
  `vim.notify` so users get a clear "not yet implemented" signal
  instead of an LSP "unknown command" error.
- `lua/nuwiki/keymaps.lua` — buffer-local default mappings. Subgroups
  (`list_editing`, `header_nav`, `diary`, `html_export`,
  `text_objects`) flip independently via the new
  `mappings.<group>` config. Heading promote/demote uses `g=`/`g-`
  to avoid clobbering Vim's built-in `=`/`-` operators.
- `lua/nuwiki/textobjects.lua` — `ah`/`ih` (around/inside heading)
  using a buffer scan for the heading-block boundary. The four
  remaining text objects from SPEC §12.10 wait until §13.1 lands
  the table/list rewriters they share infrastructure with.
- `lua/nuwiki/folding.lua` — pure regex `foldexpr` + `foldtext`
  fallback for clients without `foldingRange`. Same heading-block
  model as the server.
- `lua/nuwiki/ftplugin.lua` — single per-buffer attach entry point.
  `folding = 'lsp'` (default) uses Neovim 0.11+'s `vim.lsp.foldexpr`,
  falling back to the regex on older versions; `'expr'` forces the
  fallback; `'off'` skips folding setup.
- `lua/nuwiki/config.lua` — extends defaults with `mappings = {...}`
  (P10 keymap layer) and `folding` (P14 resolved).
- `ftplugin/nuwiki.vim` — declares every `:Vimwiki*` / `:Nuwiki*`
  command from SPEC §12.10 by inlining `lua require(...).fn()`
  bodies (no script-local function indirection so commands stay
  callable after re-source). Plain-Vim users get only the buffer
  options; they're expected to drive the LSP via vim-lsp / coc's
  built-in commands.

Health check (§12.10 additions):
- `:checkhealth nuwiki` now reports the count of `executeCommand`
  entries the server advertises, whether `foldingRange` capability
  was negotiated, and whether the configured HTML output directory
  exists + is writable. Default keymap subgroup status is also
  surfaced.

Tests: 8 new in `phase19_folding.rs` covering empty docs, single
heading → EOF, sibling headings each getting their own fold,
nested heading bounded by parent, top-level list folds, single-line
heading non-fold, fold-kind classification, and `line_count`
semantics. Total 377 tests pass; fmt + clippy clean.

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

lua/nuwiki/config.lua

@@ -1,6 +1,7 @@
 -- lua/nuwiki/config.lua — defaults and user-config merge.
 --
--- Schema mirrors SPEC §7.5. Keep this aligned with `doc/nuwiki.txt`.
+-- Schema mirrors SPEC §7.5 + §12.11. Keep this aligned with
+-- `doc/nuwiki.txt`.
 
 local M = {}
 
@@ -9,6 +10,22 @@ M.defaults = {
   file_extension = '.wiki',
   syntax         = 'vimwiki',
   log_level      = 'warn',
+
+  -- Phase 19: per-buffer glue. Each subgroup can be flipped off
+  -- independently to suppress the corresponding keymaps.
+  mappings = {
+    enabled       = true,
+    list_editing  = true,
+    header_nav    = true,
+    diary         = true,
+    html_export   = true,
+    text_objects  = true,
+  },
+
+  -- Phase 19: folding. `'lsp'` uses the server's `foldingRange`
+  -- provider (Neovim 0.11+ wires it automatically). `'expr'` falls
+  -- back to a regex-based `foldexpr`. `'off'` skips folding setup.
+  folding = 'lsp',
 }
 
 M.options = vim.deepcopy(M.defaults)