build_star_triangle(rows) that returns a multi-line string forming a left-aligned triangle of asterisks with rows rows (row i has i asterisks), built by appending each row to a list and joining with newlines at the end — the standard "string builder" pattern for avoiding repeated concatenation.rows = 3
'*\n**\n***'
Row 1 has 1 star, row 2 has 2 stars, row 3 has 3 stars, joined by newlines.
Append each row (built with `'*' * i`) to a list.
Join the list with `'\n'.join(lines)` once, at the end.