feat: add foldl_flatMap and foldr_flatMap theorems

Add theorems relating folding over flatMapped collections to nested folds
for List, Array, and Vector. These theorems enable simplifying complex fold
operations by showing that (l.flatMap f).foldl g init equals
l.foldl (fun acc x => (f x).foldl g acc) init, and similarly for foldr.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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