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-core/tests/html_renderer.rs

@@ -328,6 +328,64 @@ fn template_substitutes_vimwiki_percent_placeholders() {
     assert!(!out.contains('%'), "no placeholder left: {out}");
 }
 
+// ===== Colour spans =====
+
+// `ColorNode` is an extension point the vimwiki parser never emits, so build
+// a document around one by hand and render it directly.
+fn doc_with_color(color: &str) -> DocumentNode {
+    use nuwiki_core::ast::{ColorNode, ParagraphNode};
+    let color_node = InlineNode::Color(ColorNode {
+        span: Span::default(),
+        color: color.to_string(),
+        children: vec![InlineNode::Text(TextNode {
+            span: Span::default(),
+            content: "hi".to_string(),
+        })],
+    });
+    DocumentNode {
+        children: vec![BlockNode::Paragraph(ParagraphNode {
+            span: Span::default(),
+            children: vec![color_node],
+        })],
+        ..DocumentNode::default()
+    }
+}
+
+#[test]
+fn color_dic_name_uses_inline_style_via_default_template() {
+    let doc = doc_with_color("red");
+    let renderer =
+        HtmlRenderer::new().with_colors([("red".to_string(), "crimson".to_string())].into());
+    let out = renderer.render_to_string(&doc).unwrap();
+    assert!(
+        out.contains("<span style=\"color:crimson\">hi</span>"),
+        "got: {out}"
+    );
+}
+
+#[test]
+fn color_tag_template_override_is_honored() {
+    let doc = doc_with_color("red");
+    let renderer = HtmlRenderer::new()
+        .with_colors([("red".to_string(), "crimson".to_string())].into())
+        .with_color_template("<em data-style=\"__STYLE__\">__CONTENT__</em>");
+    let out = renderer.render_to_string(&doc).unwrap();
+    assert!(
+        out.contains("<em data-style=\"color:crimson\">hi</em>"),
+        "got: {out}"
+    );
+}
+
+#[test]
+fn unknown_color_name_falls_back_to_class() {
+    let doc = doc_with_color("plaid");
+    let out = HtmlRenderer::new().render_to_string(&doc).unwrap();
+    assert!(
+        out.contains("<span class=\"color-plaid\">hi</span>"),
+        "got: {out}"
+    );
+}
+
 // ===== End-to-end smoke =====
 
 #[test]