QuantCompressor.new_at_stage(): investigate if generation of PQ pivots is needed there

Nobody has claimed this yet.

QuantCompressor.new_at_stage() quantizes a dataset. It expects a file with PQ pivots to exist (a previous step generates it).

However, QuantCompressor.new_at_stage() contains a recovery logic which generates the PQ pivots if the pivots file is missing:

```
if !pivots_exists {
generate_pq_pivots(..);
}

Investigate if this fallback logic is useful.

Read QuantCompressor.new_at_stage() and the preceding PQ-pivot generation step, then trace callers that may rely on missing-file recovery. Done means determining whether the fallback is useful and identifying the behavior to remove or change, with the relevant validation updated.

Written by the indexing model from the issue text.