[HIGH] Data race on finishReasonSeen: read/write without synchronization while concurrent Close+Recv is an intended usage pattern

finishReasonSeen is read at line 102 (if !s.finishReasonSeen) and written at line 126 (s.finishReasonSeen = true) with no lock held, yet the existing mu comment explicitly says it protects against "a Recv that errors concurrently with the consumer's Close". The typical teardown path this PR fixes is: consumer goroutine calls Close() while another goroutine is blocked in s.inner.Recv(). Once the transport error fires, that goroutine reads finishReasonSeen at line 102. If a separate Recv call is concurrently at line 126 writing true, or if the Go memory model doesn't guarantee visibility without synchronization, this is an unsynchronized concurrent read/write — a classic data race detectable by go test -race.

All other lifecycle flags (ended, innerClosed) are already protected by s.mu. finishReasonSeen should be too. Two clean options:

// Option A: atomic bool (zero overhead, no lock widening)
finishReasonSeen atomic.Bool

// read:
if !s.finishReasonSeen.Load() {
    s.span.RecordError(err, ClassifyError(err))
}
// write:
s.finishReasonSeen.Store(true)

// Option B: hold s.mu around both the read and the write
// (read — inside the err != nil branch)
s.mu.Lock()
seen := s.finishReasonSeen
s.mu.Unlock()
if !seen {
    s.span.RecordError(err, ClassifyError(err))
}

// (write — inside the choices loop)
s.mu.Lock()
s.finishReasonSeen = true
s.mu.Unlock()

finishReasonSeen has no data race in practice. It is read and written exclusively inside Recv(), which is called by a single consumer goroutine — the reader loop in handleStream. The goroutine that runs concurrently (Close()/endOnce() via defer stream.Close()) never touches finishReasonSeen; that's precisely why ended and innerClosed need mu and this field doesn't. This matches the existing unlocked mutations in Recv() — span.MarkChunk(), SetResponseID(), AddFinishReason(), RecordUsage() — which are all single-consumer for the same reason. Adding mu to only finishReasonSeen would be inconsistent with that established pattern.