Fix ring proof domain size calculation

[prasad-kumkar]

• Align RingProofParams domain sizing with the spec/ark-vrf overhead: scalar bits plus padding rows
• Enforce max_ring_size against spec capacity and make Ring use that capacity directly
• Enable pytest-xdist parallelism in CI, tox, and dev dependencies
• Add regression coverage for spec capacity boundaries and the Safrole ring root vector

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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