Sequential Reference Active Learning Proposer

Builds a batch by repeatedly rescoring after treating already selected batch points as temporary distance references.

This requires a label-free AcqFunctionDist (such as GSx), whose scores only depend on the input-space reference points: the proposer overrides the reference points with a mix of unlabeled batch candidates and archive rows, which breaks acquisitions that align distance columns with archive labels positionally (such as iGS or IDEAL).

It may be necessary to use ALProposerSequentialReference even when proposing only a single point with this proposer itself if the proposer is part of a portfolio of proposers.

Creates a sequential-reference proposer.