feat(output): add WriterAsTraceConsumer adapter (PIPE-1024)

[Dylan-M]

Proposed Change

First PR in the PIPE-1024 traces embed migration stack. Adds the WriterAsTraceConsumer adapter in output/adapter.go, parallel to WriterAsLogConsumer and (in #229) WriterAsMetricConsumer. The adapter bridges any output.TraceWriter to an embed.TraceConsumer by pushing each span through WriteTrace.

Foundation PR — no generator wiring yet. PR 2 migrates generator/traces to construct against embed.TraceConsumer.

Tests

• TestWriterAsTraceConsumerPushesEachSpan
• TestWriterAsTraceConsumerStopsOnFirstError

Part of PIPE-1024.

[Dylan-M]

• test(embed): add traces integration test + docs update for TraceConsumer (PIPE-1024) #236
• feat(dispatch): wire traces through embed via EmbedConsumers (PIPE-1024) #235
• feat(traces): migrate to embed.TraceConsumer + ProducerModule with realistic per-span emission timing (PIPE-1024) #234
• feat(output): add WriterAsTraceConsumer adapter (PIPE-1024) #233 👈 (View in Graphite)
• main

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[eKuG]

nil writer not guarded in constructor. Calling adapter with nil TraceWriter panics on WriteTrace

[Dylan-M]

Good catch, thanks. Added a nil-guard panic at construction time in WriterAsTraceConsumer with a clear message, plus a unit test for the panic path. Also applied the same symmetric fix to WriterAsLogConsumer (identical to the one in #229's branch) so the protection lands on main whichever stack merges first — patch-id should drop the duplicate when the second one rebases.