fix(search): index changed files with srcDir-relative paths in dev

indexFile expects a srcDir-relative page and joins it with srcDir, but
hotUpdate passed the absolute path, producing a doubled path that
failed the existence check — so edits never updated the local search
index. Regression from #3374.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/node/plugins/localSearchPlugin.ts

@@ -220,7 +220,7 @@ export async function localSearchPlugin(
       if (this.environment.name !== 'client') return
 
       if (file.endsWith('.md')) {
-        await indexFile(file)
+        await indexFile(slash(path.relative(siteConfig.srcDir, file)))
         debug('🔍️ Updated', file)
         onIndexUpdated()
       }