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>

ftplugin/vimwiki.vim

@@ -1,4 +1,4 @@
-" ftplugin/nuwiki.vim — per-buffer settings + command/keymap glue.
+" ftplugin/vimwiki.vim — per-buffer settings + command/keymap glue.
 "
 " Wires up:
 "   - basic edit-time options (commentstring, suffixesadd, …)
@@ -126,7 +126,7 @@ if !has('nvim')
   command! -buffer -range VimwikiRemoveSingleCB       call nuwiki#commands#list_remove_checkbox()
   command! -buffer        VimwikiRemoveCBInList       call nuwiki#commands#list_remove_checkbox_in_list()
   command! -buffer -range -nargs=+ VimwikiListChangeLvl       call nuwiki#commands#list_change_lvl(<line1>, <line2>, <f-args>)
-  command! -buffer -nargs=? VimwikiNormalizeLink       call nuwiki#commands#normalize_link(<args>)
+  command! -buffer -nargs=? VimwikiNormalizeLink       call nuwiki#commands#normalize_link(<q-args>)
   command! -buffer        VimwikiRenumberList          call nuwiki#commands#list_renumber()
   command! -buffer        VimwikiRenumberAllLists      call nuwiki#commands#list_renumber_all()
   command! -buffer -nargs=? VimwikiTableAlignQ         call nuwiki#commands#table_align_or_cmd('gqq')
@@ -219,7 +219,7 @@ if !has('nvim')
   command! -buffer -range        NuwikiDecrementListItem      call nuwiki#commands#list_cycle_symbol_range(-1, <line1>, <line2>)
   command! -buffer -range -nargs=1 NuwikiChangeSymbol           call nuwiki#commands#list_change_symbol_range(<q-args>, <line1>, <line2>)
   command! -buffer -nargs=1 NuwikiChangeSymbolInList     call nuwiki#commands#list_change_symbol(<q-args>, 1)
-  command! -buffer -nargs=? NuwikiNormalizeLink        call nuwiki#commands#normalize_link(<args>)
+  command! -buffer -nargs=? NuwikiNormalizeLink        call nuwiki#commands#normalize_link(<q-args>)
   command! -buffer        NuwikiRenumberList           call nuwiki#commands#list_renumber()
   command! -buffer        NuwikiRenumberAllLists       call nuwiki#commands#list_renumber_all()
   command! -buffer -nargs=? NuwikiTableAlign           call nuwiki#commands#table_align_or_cmd('gqq')