phase 9: editor glue — plugin, LSP wiring, install, health, docs

P5 + P6 resolved (SPEC §11): Neovim 0.11+ and Vim 9.1+.

Universal Vim entry:
- plugin/nuwiki.vim dispatches Vim vs Neovim. Neovim path waits for
  the user's `require('nuwiki').setup()` call; Vim path starts the
  LSP client on `FileType vimwiki` and exposes `:NuwikiInstall`.
- ftdetect/nuwiki.vim associates `*.wiki` with the `vimwiki` filetype.
- ftplugin/nuwiki.vim sets commentstring + comments + formatoptions +
  suffixesadd + iskeyword, with a clean `b:undo_ftplugin`.
- syntax/nuwiki.vim ships default `@vimwiki*` highlight links for the
  semantic-token legend (with `level1..6` + `centered` modifiers),
  plus a minimal regex fallback so static highlighting works before
  the LSP attaches.

Lua glue (Neovim 0.11+):
- init.lua exposes setup() / install() / health() — wiring only per
  SPEC §6.2.
- config.lua holds the §7.5 schema (wiki_root / file_extension /
  syntax / log_level) and a tbl_deep_extend apply().
- lsp.lua registers via `vim.lsp.config{} + vim.lsp.enable` on 0.11+;
  falls back to a FileType autocmd calling `vim.lsp.start` on older
  builds. Root dir walks up for `.git` / `.nuwiki`, then uses
  wiki_root.
- install.lua does target-triple detection (Linux gnu/musl x x86_64/
  aarch64, macOS arm/x86, Windows), curl+tar download from the
  release URL, cp/chmod into bin/, with a cargo fallback. Honours
  `g:nuwiki_build_from_source`.
- health.lua implements `:checkhealth nuwiki` per §7.6.

VimL glue (Vim 9.1+):
- autoload/nuwiki/lsp.vim follows §7.4 preference order:
  vim-lsp (calls `lsp#register_server`) → coc.nvim (prints the
  coc-settings.json snippet) → error.
- scripts/download_bin.vim mirrors the Lua installer for Dein /
  vim-plug build hooks — same target triples, same download →
  fallback → cargo build chain.

Help: doc/nuwiki.txt with tags for install (lazy.nvim / vim-plug /
Dein), config options, health, LSP feature list, `:NuwikiInstall`.

Rust workspace unchanged this phase; 172 tests still green locally.

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

ftplugin/nuwiki.vim

@@ -1,2 +1,18 @@
-" ftplugin/nuwiki.vim — per-buffer filetype settings for .wiki buffers.
-" Implementation deferred to Phase 9 (Editor Glue).
+" ftplugin/nuwiki.vim — per-buffer settings for `.wiki` buffers.
+"
+" Keeps this minimal: only the options that affect editing ergonomics for
+" markup files. Anything richer (folding, navigation keymaps, …) ships
+" downstream once users opt in.
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+setlocal commentstring=%%\ %s
+setlocal comments=:%%
+setlocal formatoptions+=ron
+setlocal suffixesadd=.wiki
+setlocal iskeyword+=-
+
+let b:undo_ftplugin = 'setlocal commentstring< comments< formatoptions< suffixesadd< iskeyword<'