Fix state transfer rate before the bar starts

[jakezwang]

Before the first update, New64(100).State() computes the transfer rate as 0/0, producing NaN. This makes the state impossible to encode as JSON, and the /state handler created by StartHTTPServer returns an empty body.

Leave the rate at zero until positive elapsed time is available. Add regressions for unstarted known-length, unknown-length, and resumed bars, their transition to a positive rate, and an actual initial /state request.

Validation: full go test -race -cover -vet=off . on Go 1.25.13 and 1.26.6; go vet .; go mod tidy -diff; go build ./.... The three initial-state cases and HTTP regression fail before the fix and pass afterward.

[schollz]

Thanks!! Looks good