Issue #72: G1-G8 entropy sweep (top-3 only)

## Task
Run G1-G8 entropy sweep (top-3 only).

## Acceptance Criteria
- Run G1-G8 entropy sweep
- Top-3 results only
- Save to `experiments/entropy/g1-g8-sweep/`

## Refs
- EXP-001: Trinity physics

## Commit
"exp(entropy): G1-G8 sweep (top-3 results)"  
Ref: EXP-001