docs: add line number reference for template replacement section

Added specific line numbers (40-262) to Phase 4.3 specification
to indicate the exact section in templates/bookshelf.templ that
should be replaced with the new filter form code.

This clarifies the implementation instructions by providing precise
file location information for the filter section replacement.

UNIFIED_SEARCH_IMPLEMENTATION.md

@@ -711,7 +711,7 @@ hx-get="/api/media-items/search?library_id={ currentLibraryID }&limit={ limit }&
 
 **File:** `templates/bookshelf.templ`
 
-**Replace filter form section (includes all filters) with:**
+**Replace filter form section (includes all filters) with (lines 40-262):**
 
 ```html
 <!-- Filter Form with Autocomplete + Search Button -->