Split observability runtime by responsibility

[anth-volk]

Fixes #17

Summary

• Keep ObservabilityRuntime as the public interface and move context state, request and operation lifecycles, segments, structured logs, metrics, and tracing into focused modules.
• Split runtime and destination tests by responsibility, isolate shared test context state, and update AI-facing engineering guidance.
• Preserve public method signatures and existing behavior. All Python files in the repository are under 600 lines.

Validation

• 217 tests passed; 92% coverage.
• Ruff format and lint checks passed.
• uv build and Towncrier fragment check passed.