During the AST migration, _count_concat lost its @lru_cache (it was
replaced by an uncached recursive version). Distributing a length L
across concat parts then revisited the same (remaining_parts, length)
states exponentially -> 16M function calls for a single 4-method group,
and several RAGSAK groups took 18-52s (looked hung at the tail).
Restore memoization: _count_concat is now @lru_cache-keyed on
(tuple(parts), length). Also fix a stray blank line between the
@lru_cache decorator and count_words.
Impact (RAGSAK, --slice package --min-structure 0.5):
straggler groups 18-52s -> <0.5s; full run 54.9s -> 6.7s.
Adds a regression test asserting a length-20 (a|b)* string counts in <2s.
