Create a simple JMH benchmark to measure FST compilation / traversal times

### Description

Over in #12543 we are struggling to measure the performance cost of different ways of creating an on-heap reader/writer. We have been using the "rough" numbers coming out of `Test2BFST` runs but this is non-ideal -- it is test code, running with assertions, perhaps doing sub-optimal FST usage (not matching what, say, block tree would do to the terms index).

Let's create a simple micro-benchmark to more readily benchmark FST changes?

Start by reading the Test2BFST code and issue #12543 to understand the existing rough measurements and the reader/writer creation approaches being compared. Define a simple JMH benchmark for FST compilation and traversal, with results that avoid assertion-enabled test execution and better reflect terms-index usage.

Written by the indexing model from the issue text.