* chore: apply go fix modernizers from Go 1.26
automated refactoring: interface{} to any, slices.Contains,
and other idiomatic updates.
* feat(ci): add `go fix` check to Go analysis workflow
ensures Go 1.26 modernizers are applied, fails CI if `go fix ./...`
produces any changes (similar to existing `go fmt` enforcement)
This is the slowest test in the sharness test suite, because it has
very long sleeps. It usually takes 2+ minutes to run.
This new impl runs all peering tests in about 20 seconds, since it
polls for conditions instead of sleeping, and runs the tests in
parallel.
This also has an additional test case for a peer that was never online
and then connects.
