fix(lsp): match wikilink anchors against raw heading text

`[[Page#Heading text]]` parsed the anchor as the literal string after
`#`, but the index stored each heading's anchor as `slugify(title)`.
The two never compared equal, so every TOC-style anchor link
(`[[#1. Topologia atual]]`, etc.) was reported as a broken anchor and
goto-definition couldn't jump to the right heading either.

Slugify the request before comparing — `slugify` is idempotent on
valid-slug input, so the same code path now accepts both
`[[Page#some-heading]]` (slug form) and `[[Page#Some Heading]]`
(raw text). Applied to diagnostics, navigation, and hover preview so
all three agree on what "an anchor matches a heading" means.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/nuwiki-lsp/src/index.rs

@@ -62,6 +62,18 @@ pub struct HeadingInfo {
     pub span: Span,
 }
 
+impl IndexedPage {
+    /// Find a heading whose anchor matches `request`. The on-disk wikilink
+    /// syntax lets users write either the slug (`[[Page#some-heading]]`) or
+    /// the raw heading text (`[[Page#Some Heading]]`). `slugify` is
+    /// idempotent on valid slug input, so applying it to the request
+    /// before comparison handles both forms in a single pass.
+    pub fn find_heading_by_anchor(&self, request: &str) -> Option<&HeadingInfo> {
+        let needle = slugify(request);
+        self.headings.iter().find(|h| h.anchor == needle)
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct OutgoingLink {
     /// Resolved target page name (for `LinkKind::Wiki`) or `None` for kinds