Replace linear recursive instantiation with direct pack expansion
for 1-4 sequences, and binary tree reduction for larger cases.
Before: O(N) depth for merging N sequences
After: O(log N) depth with O(1) for up to 4 sequences
This further reduces maximum nesting depth from 26 to 22 levels
when combined with the previous sequence_gen optimization.
Co-Authored-By: Claude <noreply@anthropic.com>
