CRITICAL RUNTIME FIX: Remove tuple unpacking from environment handlers - graders now return FLOAT only, not tuples

CRITICAL FIX: Graders now return FLOAT ONLY (not tuples) for validator contract compliance - extracted float from raw grader tuples

CRITICAL FIX: Wrappers now return full tuples (reward, error, bugs) instead of just floats - fixes env.py unpacking

Phase 2: Benchmark Quality Upgrades - Progressive graders, realistic multi-bug tasks, ground_truth exposure