The handler used a line-by-line regex requiring (symbol and (lib_id on
the same line, but KiCAD's file writer places them on separate lines.
Replace with the content-string approach (already used by edit handler)
so \s+ matches across newlines. Add regression tests covering inline,
multi-line, and power symbol (#PWR) cases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
