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/diagnostics.rs

@@ -212,7 +212,8 @@ pub fn classify_link(
 }
 
 fn anchor_resolves_on(page: &crate::index::IndexedPage, anchor: &str) -> bool {
-    page.headings.iter().any(|h| h.anchor == anchor) || page.tags.iter().any(|t| t.name == anchor)
+    page.find_heading_by_anchor(anchor).is_some()
+        || page.tags.iter().any(|t| t.name == anchor)
 }
 
 /// Same classification as [`classify_link`] but driven from the cached