Leetcode 1622. Fancy Sequence

Support append, addAll, multAll, and getIndex on a sequence under up to 1e5 operations, where addAll/multAll affect all existing elements and results are modulo 1e9+7. The core challenge is to maintain and compose global affine transformations (value = a*orig + b mod MOD) and use per-element snapshots (with modular inverses) to answer point queries efficiently.