Add support for reconnect for state with 1B accounts

This epic covers the changes required to support reconnect of state with 1 billion accounts (~6 billion entries in the virtual map, ~2.9 billion leaf paths). The current implementation does not converge: each reconnect iteration takes ~350–400s, exceeding the ~180s gossip threshold, so the learner re-enters reconnect indefinitely.

Root-cause investigation identified a stack of independent bottlenecks in the learner's apply path and the pull protocol, each of which must be addressed for reconnect to complete within the gossip window.