Add challenge 74: N-body Gravitational Force (Medium)

[claude]

Summary

• Adds challenge 74: N-body Gravitational Force (Medium difficulty)
• Teaches the classic all-pairs O(N²) GPU computation pattern — shared-memory tiling is the key optimization to escape global-memory bandwidth bottleneck
• Softened gravitational force formula: F_i = Σ_{j≠i} m_j (r_j - r_i) / (|r_j - r_i|² + ε²)^{3/2} with ε = 1e-3
• Inputs: positions (N×3 float32), masses (N float32); Output: forces (N×3 float32)
• Performance test: N = 8,192 (~67M pairwise force evaluations)

Why this challenge?

Unlike the many element-wise challenges already in the repo, this is a genuine all-pairs computation where every output depends on all N inputs. The natural optimization (shared-memory tiling to amortize global loads across a block of particles) is the same conceptual leap as GEMM tiling but applied to a completely different physics problem — making it educational without being a duplicate.

Test plan

• pre-commit run --all-files passes (Black, isort, flake8, clang-format)
• run_challenge.py --action run passes on NVIDIA TESLA T4 (example test)
• All checklist items from CLAUDE.md verified:
  • challenge.html starts with <p>, uses <h2> sections, first example matches generate_example_test(), LaTeX bmatrix used consistently, performance bullet present
  • challenge.py: inherits ChallengeBase, all 6 methods, 9 functional tests (edge/pow2/non-pow2/realistic + zero inputs + negatives), performance test fits 5× in 16 GB
  • All 6 starter files present with correct parameter description comments
  • Solution file not committed

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