Add implementation spec for segment ranking test suite

Detailed implementation spec covering:
- tub_test_data_generator.py: DrivingProfile dataclass, 8 pre-defined
  profiles, deterministic sensor record generation with correlated
  values, create_multilap_tub() and create_multilap_tub_via_writer()
  factory functions
- test_segment_ranking_comprehensive.py: 30 tests across 7 classes
  with exact setup/assert pseudocode for each test
- Risk register for non-deterministic timestamps, None handling,
  median implementation details

Based on PRD in docs/prd_segment_ranking_tests.md

https://claude.ai/code/session_01Q6cz4PnXVXz8sPX187A2xM

Author: claude