refactor(compact): store tool-pairing balance once per surface cut

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.

packages/compact/compact/tests/tool-pairing.spec.ts

@@ -131,7 +131,7 @@ describe('tool-pairing surface identity', () => {
     expect(() => toolPairingBalancedAfter(session, staleTail)).toThrow(/surface seq .* not found/)
   })
 
-  it('uses the cached positional successor instead of a caller node next field', () => {
+  it('ignores a caller-held node next field and answers from cached balances', () => {
     const session = closedToolStep()
     const assistant = nodeAt(session, seqOf(session, 'assistant/message'))
     expect(toolPairingBalancedAfter(session, { ...assistant, next: null })).toBe(false)