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/index.rs

@@ -123,6 +123,18 @@ pub struct WorkspaceIndex {
     /// the nav handlers (tag-as-anchor `definition`, `workspace/symbol`
     /// inclusion) and the `nuwiki.tags.search` command.
     pub tags_by_name: HashMap<String, Vec<TagOccurrence>>,
+    /// For each source URI, the backlink-bucket keys and tag-bucket names it
+    /// contributed. Lets `remove` touch only the affected buckets instead of
+    /// scanning every bucket in the workspace (O(touched), not O(total)).
+    contributions: HashMap<Url, SourceContributions>,
+}
+
+/// The bucket keys a single source page wrote into, so they can be undone
+/// in `remove` without a full scan. See [`WorkspaceIndex::contributions`].
+#[derive(Debug, Default)]
+struct SourceContributions {
+    backlink_keys: Vec<String>,
+    tag_names: Vec<String>,
 }
 
 impl WorkspaceIndex {
@@ -175,15 +187,20 @@ impl WorkspaceIndex {
         };
         index_blocks(&ast.children, &mut page);
 
+        let mut contrib = SourceContributions::default();
         let ext = self.file_extension.as_deref();
         for link in &page.outgoing {
             if let Some(tgt) = &link.target_page {
                 let key = canonical_link_name(tgt, &name, link.is_absolute, ext);
-                self.backlinks.entry(key).or_default().push(Backlink {
-                    source_uri: uri.clone(),
-                    source_span: link.span,
-                    target_anchor: link.anchor.clone(),
-                });
+                self.backlinks
+                    .entry(key.clone())
+                    .or_default()
+                    .push(Backlink {
+                        source_uri: uri.clone(),
+                        source_span: link.span,
+                        target_anchor: link.anchor.clone(),
+                    });
+                contrib.backlink_keys.push(key);
             }
         }
 
@@ -197,7 +214,9 @@ impl WorkspaceIndex {
                     span: tag.span,
                     scope: tag.scope,
                 });
+            contrib.tag_names.push(tag.name.clone());
         }
+        self.contributions.insert(uri.clone(), contrib);
 
         self.pages_by_name.insert(name, uri.clone());
         self.pages_by_uri.insert(uri, page);
@@ -207,14 +226,26 @@ impl WorkspaceIndex {
         if let Some(page) = self.pages_by_uri.remove(uri) {
             self.pages_by_name.remove(&page.name);
         }
-        for entries in self.backlinks.values_mut() {
-            entries.retain(|b| &b.source_uri != uri);
+        // Only revisit the buckets this source actually wrote into, instead
+        // of scanning every backlink/tag bucket in the workspace.
+        if let Some(contrib) = self.contributions.remove(uri) {
+            for key in contrib.backlink_keys {
+                if let Some(entries) = self.backlinks.get_mut(&key) {
+                    entries.retain(|b| &b.source_uri != uri);
+                    if entries.is_empty() {
+                        self.backlinks.remove(&key);
+                    }
+                }
+            }
+            for name in contrib.tag_names {
+                if let Some(entries) = self.tags_by_name.get_mut(&name) {
+                    entries.retain(|t| &t.uri != uri);
+                    if entries.is_empty() {
+                        self.tags_by_name.remove(&name);
+                    }
+                }
+            }
         }
-        self.backlinks.retain(|_, v| !v.is_empty());
-        for entries in self.tags_by_name.values_mut() {
-            entries.retain(|t| &t.uri != uri);
-        }
-        self.tags_by_name.retain(|_, v| !v.is_empty());
     }
 
     /// All occurrences of a tag across the workspace. Empty slice when the
@@ -306,21 +337,6 @@ impl WorkspaceIndex {
     }
 }
 
-/// Return a parsed [`DiaryDate`] when `uri` is a daily diary entry —
-/// the path lives under `<root>/<diary_rel_path>/` and its stem parses
-/// as `YYYY-MM-DD`. For weekly/monthly/yearly entries see
-/// [`diary_period_for_uri`].
-pub fn diary_date_for_uri(
-    uri: &Url,
-    root: Option<&Path>,
-    diary_rel_path: Option<&str>,
-) -> Option<DiaryDate> {
-    match diary_period_for_uri(uri, root, diary_rel_path)? {
-        nuwiki_core::date::DiaryPeriod::Day(d) => Some(d),
-        _ => None,
-    }
-}
-
 /// Return a parsed [`DiaryPeriod`] when `uri` is a diary entry at any of
 /// the four supported frequencies. Recognises:
 ///
@@ -329,9 +345,8 @@ pub fn diary_date_for_uri(
 ///   `YYYY-MM`       → Month
 ///   `YYYY`          → Year
 ///
-/// As with `diary_date_for_uri`, the path must sit under
-/// `<root>/<diary_rel_path>/` to be accepted; without a `root` we accept
-/// any stem (so ad-hoc test setups still work).
+/// The path must sit under `<root>/<diary_rel_path>/` to be accepted;
+/// without a `root` we accept any stem (so ad-hoc test setups still work).
 pub fn diary_period_for_uri(
     uri: &Url,
     root: Option<&Path>,