Perf: Use SHA256.HashData for seeded key derivation

[MariusStorhaug]

Summary

Refactor seeded keypair path to use static SHA256.HashData APIs rather than per-call SHA256.Create()/Dispose().

Why

Static hashing APIs reduce allocations and simplify the hot-path seeded key generation code.

Proposed change

• Replace disposable SHA256 instance creation in New-SodiumKeyPair -Seed with SHA256.HashData.
• Preserve deterministic seed->keypair behavior exactly.
• Keep sensitive data clearing behavior for intermediate buffers.

Acceptance criteria

• Deterministic seeded output remains unchanged for same input seed.
• Existing tests pass.
• Seeded keypair benchmark shows reduced overhead or no regression.
• Code path has fewer transient allocations.