[thrawn01]

Purpose

Follow-up to #28. A caller who passes Options.Logger without a Transport expects the transport's "Listening on…" and server-error messages in that logger. Two paths still sent them to slog.Default():

• groupcache.New() built its default transport with empty options.
• cluster.StartWith built the parent transport with empty options. Daemon 1 got the Use provided logger in daemon constructor #28 default, but daemons 2..N come from parent.New(), which copies the parent's empty options.

cluster.Start(ctx, 3, groupcache.Options{Logger: logger})
// before: only daemon 1 logged to `logger`
// after:  all 3 daemons log "Listening on ..." to `logger`

With no logger the behavior is unchanged (slog.Default()). cmd/server/main.go is untouched, as it already uses slog.Default().

Implementation

• instance.go: default transport gets Logger: opts.Logger.
• cluster/cluster.go: parent transport gets Logger: opts.Logger.
• cluster/cluster_test.go: TestStartLogsEveryDaemonToLogger starts 3 daemons with a capturing logger and expects 3 "Listening on" lines. It failed before the fix (never reached 3) and passes after.

Verified with go vet ./... and go test -p 1 -race ./... (all pass). Without -p 1, packages collide on the fixed test ports, which predates this change.

Not verified: the New() change has no test. The default transport is not reachable from Instance, so no surface test can observe its logger. It is the same one-line change as daemon.go. No sub-agent review was run, only my own read of the diff.

Fixes ENG-580