Specialize arc store for continuous label in FST

### Description

Today we pos arcs with three strategies:

* **Direct addressing**: pos arc with bit count
* **Binary search**: pos arc with binary search
* **Linear search**: pos arc with linear search

I wonder if we can specialize the dealing the continuous label (`max_label - min_label + 1 == arc_count`) so we can pos the arc directly with `label_to_pos - min_label` ? This may be a common case for increment numeric ids or so.

Start by locating the FST arc-storage implementation and the existing direct-addressing, binary-search, and linear-search strategies described in the issue. Compare how continuous labels are detected and positioned, then validate the specialized path against the existing behavior and relevant FST tests or benchmarks.

Written by the indexing model from the issue text.