Multiplex native stream follows over shared node sessions
Restacked with signed normal merges through main at c86b6568ff1fe46655b9c00c82103d253f4dd458, including #679 and #687. The resulting tree exactly equals that main revision: the session and follow work in this PR has already landed through #557, #674, #679 and #687. There is no remaining source diff to merge.
Validation
Windows lb.ps1 cargo check --locked -p acyclic-stream --features local,http --lib --tests passes in 4.81 s. git diff --quiet c86b6568ff1fe46655b9c00c82103d253f4dd458 HEAD succeeds. No Linux rebuild was needed for an empty diff; the underlying session validation is recorded in the merged PRs.
Live steps
The abandoned-reconnect fix is merged in #687. Checked follow adapters remain in #650; the 10k-agent scenario and its latency measurements remain in #644.