pace throughput mode offered rate with --rate

[fabracht]

Adds an offered-rate control to throughput mode. The flag already existed for latency and hol-blocking, but the throughput generator never read it, so throughput mode could only saturate.

--rate 0 (the default) is unchanged: publishers offer as fast as the in-flight window allows. A positive value paces them to that aggregate rate using an absolute-deadline schedule per worker — each worker sleeps until its next scheduled instant rather than sleeping a fixed interval, so a slow publish does not permanently shift the schedule, and the pacing does not drift.

Why

Saturating a broker with QoS 1 does not measure delivery throughput: the excess is queued rather than shed, so the broker accumulates a backlog and the per-second delivered series is a ramp rather than a steady state. Averaging that ramp understates the sustained rate substantially. Pacing the offered rate below the delivery capacity produces a flat series that the existing trimmed-mean estimator summarises correctly.

Measured

Local run, 16 publisher connections to 8 subscriber connections, QoS 1, 256-byte payloads:

	saturating	--rate 10000
offered rate	uncontrolled	10,002 (100.0% of target)
delivered per-second series	0, 0, 0, 0, 0, 15222, 168638, …	80000, 79092, 80388, 80101, 80099
broker RSS	933 MB	18 MB

The paced series is flat from the first sample at 10,000 x 8 subscribers, and no backlog forms.