…porky enough to stand on its own and data_access is a port of a Java test file anyway.

- `LinearIterator` no longer aborts one step too early in the final bucket.
- `PickyIterator` now returns metadata when it picks. This allows `IterationValue` to provide better data about the current iteration progress without introducing a separate stage in the `PickyIterator` lifecycle to query about what was just picked.
- count since last iteration is now reset every iteration, making it less prone to overflow.
- end-of-histogram is detected by comparing with max nonzero index, not total count, which avoids overflow.

- Supply count at current index to `pick()` since we already have that available
- Quantile iterator won't get stuck asymptotically chasing quantile 1.0_f64
- More tests