Add segment aggregation and static FastAPI attributes

[anth-volk]

Summary

Adds the package-level observability primitives needed by downstream services to avoid custom app-owned observability plumbing:

• accumulates repeated segment timings in request/operation logs instead of overwriting the previous value
• adds timing_counts to request and operation log records
• makes set_attribute() update explicit operations nested inside request contexts
• adds public mark_ttft_attribute() for TTFT-as-log-attribute use cases
• adds FastAPI static_attributes support so apps can attach platform/runtime metadata without custom middleware
• clears stale ContextVar state when swapping the configured global runtime

Tests

• uv run pytest tests/test_runtime.py tests/test_fastapi_adapter.py tests/test_public_api.py
• 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 — 91% coverage