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>

crates/nuwiki-lsp/src/export.rs

@@ -16,13 +16,13 @@
 //!   above wired up; returns the final HTML string.
 
 use std::collections::HashMap;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 use nuwiki_core::ast::{BlockNode, DocumentNode, HeadingNode, InlineNode, LinkKind};
 use nuwiki_core::date::DiaryDate;
 use nuwiki_core::render::{HtmlRenderer, Renderer};
 
-use crate::config::{HtmlConfig, WikiConfig};
+use crate::config::HtmlConfig;
 
 /// Map a page name (e.g. `"diary/2026-05-11"`) to its on-disk HTML
 /// output path under `html_path`. Honours
@@ -251,6 +251,9 @@ pub fn render_page_html(
     if !cfg.color_dic.is_empty() {
         r = r.with_colors(cfg.color_dic.clone());
     }
+    if !cfg.color_tag_template.is_empty() {
+        r = r.with_color_template(cfg.color_tag_template.clone());
+    }
     if cfg.html_header_numbering > 0 {
         r = r.with_header_numbering(
             cfg.html_header_numbering,
@@ -425,9 +428,3 @@ code{background:#f4f4f4;padding:.1em .25em;border-radius:3px}\
 table{border-collapse:collapse}\
 table td,table th{border:1px solid #ccc;padding:.25em .5em}\
 ul.toc{padding-left:1.5em}\n";
-
-/// Convenience wrapper exposed for the dispatcher: convert a wiki config
-/// to the absolute output dir.
-pub fn html_output_dir(cfg: &WikiConfig) -> &Path {
-    &cfg.html.html_path
-}