Module Granary_ivm.JoinSource

Incremental equi-join over Z-sets (#417 Phase 2).

Join is bilinear, so its delta is Δ(L ⋈ R) = ΔL ⋈ R + L ⋈ ΔR + ΔL ⋈ ΔR. An operator value maintains the integrated inputs L and R (indexed by join key) and, given a delta on either side, returns the matching output delta and folds it into a materialized output — so the join is kept current in time proportional to the change, not the size of L and R.

Output carries bag multiplicity: a matched pair contributes the product of its input weights. Set / SELECT DISTINCT semantics, if wanted, are a Zset.S.distinct applied to the output, not built in here.