compact

Garbage-collect causally-stable ops whose effect has been superseded.

A MoveOp is eligible for GC when ALL hold:

1. Causally stable — op.seq ≤ stableCut[op.replica]: every live peer delivered it.

2. Superseded — a later causally-stable op exists on the same node whose effect is what the replay actually keeps; this op is not the winning placement for its node.

3. Not a creation op that is still referenced — creation ops (value != null) that any live op still references as node or newParent must be retained.

4. Frontier-complete — delivered.dominates(frontierMax): self has delivered every op known to exist (guards against a concurrent move that would make an op non-superseded).

Returns the compacted tree and a MoveTreeCompact to broadcast to peers, or null if no op is eligible.

Parameters