Add round_003: deflection minimization

[PunchTheDev]

Summary

Adds a third competition round targeting absolute structural stiffness: minimize tip deflection under load. Completely different optimization target from round_001 (mass) and round_002 (stiffness/weight).

Why deflection minimization is interesting

Round 001 rewards light designs; round 002 rewards light+stiff designs. Round 003 rewards just stiff — agents can use as much mass as allowed. High-modulus materials (Al 6061, SS 316) have a decisive advantage. Winning strategies maximize moment of inertia and use efficient load paths, producing designs that look nothing like rounds 1/2.

Changes

• benchmark/evaluate.py: adds "deflection_mm": "minimize" to METRICS; _compute_score returns fea_result.max_displacement_mm for this metric
• specs/generator.py: adds _baseline_deflection_mm() (Euler-Bernoulli solid-beam upper bound); generate() handles deflection_mm scoring block; --metric CLI flag updated
• specs/round_003/: 15 specs — 5 easy (PLA/PETG), 6 medium (+Al 6061), 4 hard (+SS 316). Seeds 700/800/900
• rounds/round_003.json: round definition, status upcoming
• tests/test_generator.py: 5 new tests for deflection_mm generation and round_003 spec validity