perf(addon-search): cache lowercased line text in line cache

Lazily store a lowercased line variant in the line cache entry to avoid
repeated toLowerCase calls during case-insensitive searches.

Co-authored-by: Cursor <cursoragent@cursor.com>

addons/addon-search/src/SearchLineCache.ts

@@ -15,7 +15,11 @@ export type LineCacheEntry = [
   /**
    * The offsets where each line starts when the entry describes a wrapped line.
    */
-  lineOffsets: number[]
+  lineOffsets: number[],
+  /**
+   * Lowercased line text for case-insensitive searches, lazily populated.
+   */
+  lineAsLower?: string
 ];
 
 /**