toolPairingBalancedAfter previously answered by resolving a cached
positional successor and reading its before-balance, with a null-successor
depth fallback. Both queries are the same prefix property sampled at
adjacent cuts, so the cache now holds one per-cut balance sequence
(N nodes -> N+1 cuts) plus a seq->position index; before/after differ
only by a cut offset. The successor map, the duplicate rebuild/extend
fold loops, and the non-null assertion are gone, and the running counter
is named inProgressToolCalls. Docs describing the successor mechanism
are updated in place.
