fix(lsp): resolve wikilinks that include the .wiki file extension

`[[page.wiki]]` should resolve identically to `[[page]]`. The index
keyed by stem, so links with the explicit extension fell through to
the broken-link diagnostic and to-page navigation failed. Strip the
configured extension before every page lookup (resolve, definition,
backlinks, classify_link, classify_outgoing) and stop appending the
extension in synthesise_page_uri when it's already present.

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

crates/nuwiki-lsp/src/diagnostics.rs

@@ -166,7 +166,8 @@ pub fn classify_link(
     match link.target.kind {
         LinkKind::Wiki => {
             let path = link.target.path.as_deref()?;
-            let page = index.page_by_name(path);
+            let normalized = index.strip_link_extension(path);
+            let page = index.page_by_name(normalized);
             match page {
                 None => Some(BrokenLinkKind::MissingPage(path.to_string())),
                 Some(page) => {
@@ -227,7 +228,8 @@ pub fn classify_outgoing(
     match link.kind {
         LinkKind::Wiki => {
             let path = link.target_page.as_deref()?;
-            match index.page_by_name(path) {
+            let normalized = index.strip_link_extension(path);
+            match index.page_by_name(normalized) {
                 None => Some(BrokenLinkKind::MissingPage(path.to_string())),
                 Some(page) => {
                     if let Some(anchor) = &link.anchor {