Add nested segment timing logs

[anth-volk]

Fixes #15

Summary

• Add ordered nested segment_tree timing logs for request and operation records.
• Preserve flat inclusive timing aggregates while keeping repeated sibling segments as separate entries.
• Ensure core structured log fields override caller-provided attributes.

Validation

• uv run --extra dev ruff format .
• uv run --extra dev ruff format --check .
• uv run --extra dev ruff check .
• uv run --extra dev --extra all coverage run -m pytest
• uv run --extra dev --extra all coverage report
• uv run --extra dev towncrier check --compare-with origin/main
• uv build