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/src/syntax/vimwiki/parser.rs

@@ -37,9 +37,9 @@ use crate::ast::{
     BlockNode, BlockquoteNode, BoldNode, CodeNode, CommentNode, DefinitionItemNode,
     DefinitionListNode, DocumentNode, ErrorNode, ExternalLinkNode, HeadingNode, HorizontalRuleNode,
     InlineNode, ItalicNode, KeywordNode, LinkKind, LinkTarget, ListItemNode, ListNode, ListSymbol,
-    MathBlockNode, MathInlineNode, PageMetadata, ParagraphNode, PreformattedNode, RawUrlNode, Span,
-    StrikethroughNode, SubscriptNode, SuperscriptNode, TableCellNode, TableNode, TableRowNode,
-    SoftBreakNode, TagNode, TagScope, TextNode, TransclusionNode, WikiLinkNode,
+    MathBlockNode, MathInlineNode, PageMetadata, ParagraphNode, PreformattedNode, RawUrlNode,
+    SoftBreakNode, Span, StrikethroughNode, SubscriptNode, SuperscriptNode, TableCellNode,
+    TableNode, TableRowNode, TagNode, TagScope, TextNode, TransclusionNode, WikiLinkNode,
 };
 use crate::syntax::{Parser, TokenStream};
 
@@ -1426,30 +1426,9 @@ fn strip_directory(s: &str) -> (String, bool) {
 // ===== Span helpers =====
 
 fn span_start_of_inline(node: &InlineNode) -> Option<crate::ast::Position> {
-    Some(span_of_inline(node).start)
+    Some(node.span().start)
 }
 
 fn span_end_of_inline(node: &InlineNode) -> Option<crate::ast::Position> {
-    Some(span_of_inline(node).end)
-}
-
-fn span_of_inline(node: &InlineNode) -> Span {
-    match node {
-        InlineNode::Text(n) => n.span,
-        InlineNode::Bold(n) => n.span,
-        InlineNode::Italic(n) => n.span,
-        InlineNode::BoldItalic(n) => n.span,
-        InlineNode::Strikethrough(n) => n.span,
-        InlineNode::Code(n) => n.span,
-        InlineNode::Superscript(n) => n.span,
-        InlineNode::Subscript(n) => n.span,
-        InlineNode::MathInline(n) => n.span,
-        InlineNode::Keyword(n) => n.span,
-        InlineNode::Color(n) => n.span,
-        InlineNode::WikiLink(n) => n.span,
-        InlineNode::ExternalLink(n) => n.span,
-        InlineNode::Transclusion(n) => n.span,
-        InlineNode::RawUrl(n) => n.span,
-        InlineNode::SoftBreak(n) => n.span,
-    }
+    Some(node.span().end)
 }