Add Traject Runner and diagnostic artifact in Zig 0.16

[suissa]

Summary

• Add a Zig 0.16 Traject Runner for intent.buy-product.
• Model the Customer → Sales → Financial → Stock → Delivery entities and causal step links.
• Emit a JSON Trajectory Artifact with entity snapshots, expected/observed values, and the first divergence.
• Add a controlled stock-divergence test (expected 8, observed 9) while preserving the downstream Delivery step.
• Add GitHub Actions CI and upload the generated artifact.

Validation

• Zig 0.16 tests and artifact validation passed in workflow run 39225127234.
• Artifact: traject-buy-product-stock-divergence.

The stock defect is intentionally injected in the fixture to verify diagnostic behavior; this does not claim a real inventory integration.