Add logging integration to request layer

[guidefari]

Summary

Add structured logging alongside existing span-based tracing in the request layer.

Context

PRD user stories #26, #27: observe requests/failures, instrumentation near request layer.

Current state: comprehensive span attributes via Effect.withSpan() and annotateCurrentSpan(). No structured log output for request lifecycle events (start, success, failure, retry).

Acceptance criteria

• Request lifecycle events logged (configurable verbosity)
• Log output integrates with Effect's logging infrastructure
• Logging opt-in, not forced on consumers

Related

PRD #9 — user stories 26, 27

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Migrated from guidefari/spotify-effect#46