Add PRD for comprehensive segment ranking test suite

Defines 30 new tests across 7 test classes to cover the gaps in
segment-based ranking test coverage: all 6 aggregation methods,
multi-field ranking priority, driving behavior discrimination with
correlated sensors, transform functions, TubWriter integration,
on-the-fly segment computation, and edge cases.

Includes gap analysis against existing 16 test files to avoid
duplication.

https://claude.ai/code/session_01Q6cz4PnXVXz8sPX187A2xM

Author: claude