fix(review): close all 2026-06-03 codebase-review findings (R1-R18)

Resolves the 18 findings from the parallel codebase review, tracked in
development/vimwiki-gap.md.

Correctness / perf:
- Wiki.config -> Arc<WikiConfig> so cloning a Wiki is a refcount bump (R5)
- WorkspaceIndex::remove is no longer O(n^2): a per-source contributions
  map limits the scan to buckets the source actually wrote into (R6)
- render_color now expands color_tag_template (__STYLE__/__CONTENT__),
  consuming the previously-dead field; ColorNode documented as an
  extension point; 3 renderer tests added (R3/R4)
- wiki_root_for returns empty/nil on no-match instead of falling back to
  the first wiki (R2); auto_header honours links_space_char (R7)

Cleanup / dedup:
- Remove dead #[allow(dead_code)] stubs + uncalled pub helpers, narrow
  imports (R10/R11)
- Dedup span_of_inline x3 -> InlineNode::span() (R12)
- diary_step single read lock; page_captions single pass (R13)
- Lua auto_header loop -> wiki_list(); detect_current_symbol cleanup
  (R14/R18)

Client / docs:
- :VimwikiNormalizeLink Vim cmds -> <q-args> (R17); ftplugin header fix
  (R16); vars.vim multi-wiki limitation documented (R15)
- Document 19 config options in README.md + doc/nuwiki.txt; fix
  list_margin/shiftwidth doc and stale comments (R1/R9)
- R8 investigated, confirmed not a real bug (documented)

Verified: Neovim harness 307, Vim harness 301/18/21, Rust suite 568, all
0 failed; clippy clean; fresh parallel-agent audit found no regressions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

autoload/vimwiki/vars.vim

@@ -13,6 +13,14 @@
 "   is_temporary_wiki — always 0 for a configured wiki
 "   ext / extension — file extension (default '.wiki')
 "   syntax          — syntax name (default 'vimwiki')
+"
+" LIMITATION: the optional second argument (upstream's wiki index) is
+" ignored — values always come from the scalar `g:nuwiki_*` vars, i.e.
+" the first/shorthand wiki. Third-party plugins that query a specific
+" wiki by index in a multi-wiki setup will get the first wiki's values.
+" The shim exists for single-wiki compatibility (vimwiki-sync, vim-zettel);
+" extend here with `g:nuwiki_wikis[a:1]` lookups if multi-wiki support is
+" needed.
 function! vimwiki#vars#get_wikilocal(key, ...) abort
   if a:key ==# 'path'
     let l:root = expand(get(g:, 'nuwiki_wiki_root', '~/vimwiki'))