fix(precompute): route SetAggregator and DeltaSetAggregator, preserve deltas

[milindsrivastava1997]

Summary

Closes #477

• Route SetAggregator and DeltaSetAggregator to keyed updaters instead of silently falling back to Sum.
• Preserve DeltaSetAggregator added/removed key differences across windows.
• Drop ForwardToStore samples for DeltaSetAggregator because append-only late corrections cannot preserve stateful deltas.
• Fix first-batch, multi-window watermark handling so on-time samples are not treated as evicted.
• Reject unknown subpopulation aggregation subtypes instead of silently defaulting.

Verification

• cargo test --workspace -- --test-threads=1
• Pre-commit formatting, check, clippy, and test hooks pass.