agent: index connections with active subscriptions instead of folding all subscriptions per batch

[Narasimha-sc]

Resubscribing a session rebuilds a Set ConnId covering all of that session's active subscriptions, once per batch, just to answer "was this connection already subscribed" for at most subsBatchSize connections.

Since activeSubs grows as the resubscription proceeds, resubscribing N queues in one session costs on the order of N²/2700 traversals and allocates a set of up to N elements per batch — tens of MB of short-lived allocation per session, repeated on every network change, which under the non-moving collector shows up as resident memory rather than only CPU.

SessSubs now keeps activeConns :: TMap ConnId Int in sync with activeSubs (counted, since a connection can hold several queues), maintained at each of the five mutation sites. Behaviour is unchanged. Plan in plans/2026-09-26-index-active-sub-conns.md.