Add weighted aggregation strategy for Suite() (including nested weights)

## Question
How to support **weighted aggregation** in `Suite()`?

Current strategies (`average`, `average_of_averages`, etc.) don’t express explicit proportions across domains and subdomains. For example, suppose:

Top-level mix:
- code: 40%
- QA: 30%
- math: 30%

Nested mix inside code:
- python: 50%
- rust: 30%
- c: 20%

