idsToTruncate

Returns the set of element ids to drop from the sequence on this compaction pass.

sequence is the full ordered sequence of all RgaIds (both visible and tombstoned). tombstones is the subset that have been removed but not yet compacted.

The returned ids are dropped on this pass via the history-windowing path, which is un-gated — unlike tombstone GC it does not pass through the causal-stability barrier. Windowing forgets position, so dropping a live id is safe: reroot-to-HEAD (#254) keeps the retained window reachable, and a concurrent Insert(J, after=window-dropped-I) resurfaces at the window boundary instead of being orphaned. The barrier-gated tombstone-GC path stays separate and position-preserving; the two never share a gate.