feat(corpus): category filter, passage expand, drill enhancements, URL hash state

- Search: category filter pills scope results to a legal domain
- Search: full chunk text returned; click to expand inline beyond 600-char excerpt
- Drill panel: total count label ("Showing X of Y"), sort dropdown, title filter (300ms debounce)
- URL hash: preserves query/mode/lang/category/drill state for bookmarking

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

api/corpus-documents.php

@@ -12,8 +12,15 @@ try {
 
     $category   = isset($_GET['category'])    && $_GET['category']    !== '' ? trim((string)$_GET['category'])    : null;
     $sourceName = isset($_GET['source_name']) && $_GET['source_name'] !== '' ? trim((string)$_GET['source_name']) : null;
+    $titleFilter = isset($_GET['title'])      && $_GET['title']       !== '' ? trim((string)$_GET['title'])       : null;
     $offset     = max(0, (int)($_GET['offset'] ?? 0));
     $limit      = max(1, min(50, (int)($_GET['limit'] ?? 20)));
+    $orderBy    = match($_GET['sort'] ?? 'newest') {
+        'oldest' => 'd.updated_at ASC',
+        'alpha'  => 'd.title ASC',
+        'chunks' => 'chunk_count DESC',
+        default  => 'd.updated_at DESC',
+    };
 
     // Build WHERE clause
     $where  = ["d.corpus_id = 1", "d.status = 'ready'"];
@@ -24,6 +31,11 @@ try {
         $params[] = $category;
     }
 
+    if ($titleFilter !== null) {
+        $where[]  = 'd.title LIKE ?';
+        $params[] = '%' . str_replace(['%', '_'], ['\\%', '\\_'], $titleFilter) . '%';
+    }
+
     if ($sourceName !== null) {
         // Filter by source via a JOIN to corpus_sources on category match
         // or by matching the scraper's URL pattern in source_url
@@ -61,7 +73,7 @@ try {
          LEFT JOIN chunks c ON c.document_id = d.id
          WHERE $whereStr
          GROUP BY d.id
-         ORDER BY d.updated_at DESC
+         ORDER BY $orderBy
          LIMIT $limit OFFSET $offset"
     );
     $dataStmt->execute($params);
@@ -82,6 +94,8 @@ try {
         'filter'    => [
             'category'    => $category,
             'source_name' => $sourceName,
+            'title'       => $titleFilter,
+            'sort'        => $_GET['sort'] ?? 'newest',
         ],
     ]);
 } catch (Throwable $e) {