Optimize _byte_pair_merge to o(m log n) using heap-based candidate selection

[ArchishmanSengupta]

Summary

-> Replaced the O(m·n) sequential merge scan with a heap-driven algorithm that maintains candidate merges in a max-heap keyed by rank, updating only local neighbors on each merge.

-> This yields m·log n behavior where:
m: number of merges and
n: is the number of initial symbols

Key changes:

1. BinaryHeap and candidate struct in _byte_pair_merge, maintaining a linked-list of live nodes and per-position versions to avoid stale heap entries.
2. Computes local ranks via compute_rank_at and updates only affected neighbors after each merge.
3. Added targeted unit tests for _byte_pair_merge boundaries.

Complexity:

Before: repeated linear scans → approximately O(m·n) in worst-case merges.
After: heap operations per merge → O(m·log n), with O(n) initialization.