`MultiplexInstrument` does not fan out to all tracers

`MultiplexInstrument` can hold multiple `Tracer`s, but only sends traces to one of them — is this expected?

The asymmetry is that `inject`/`extract` fan out to every member of the multiplex, but span creation doesn't: `InstrumentationSystem.tracer`/`legacyTracer` and the free-function `withSpan`/`startSpan` resolve to the first `Tracer` in the
multiplex and ignore the rest. So context is propagated through all members, but only one of them ever receives spans.

```
import Tracing
import InMemoryTracing

let tracerA = InMemoryTracer()
let tracerB = InMemoryTracer()
InstrumentationSystem.bootstrap(MultiplexInstrument([tracerA, tracerB]))

withSpan("request") { _ in }

print(tracerA.finishedSpans.count) // 1
print(tracerB.finishedSpans.count) // 0 ← tracerB never sees the span
```

So my questions:

Is "only the first `Tracer` receives spans" the intended contract? If so, it would be worth documenting on `MultiplexInstrument`, since "use multiple instruments" reads as though all of them are active. If not, should span creation fan out to every `Tracer` in the multiplex?

Inspect MultiplexInstrument and the InstrumentationSystem.tracer/legacyTracer, withSpan, and startSpan paths named in the report. Determine whether the contract is first-tracer-only or fan-out, then document or implement the chosen behavior and add coverage showing how both tracers receive spans.

Written by the indexing model from the issue text.