fix: Use thread-safe hash for big segments hashing.

[kinyoklion]

SHA256 instances are not thread-safe and under heavy-parallelism will fail.
Newer .Net frameworks have a static method which is higher performance under heavy-parallelism and inherently thread-safe.

This PR introduces a thin abstraction that uses the correct method in a thread-safe way depending on the framework.

---

Note

Replaces a shared SHA256 instance with a thread-safe LdSha256 utility and updates Big Segments hashing to use it.

• Internal hashing:
  • Add LdSha256 utility that uses SHA256.HashData on .NET 5+ and per-call ComputeHash on older targets.
  • Update BigSegmentsInternalTypes.BigSegmentContextKeyHash to use LdSha256.HashData instead of a shared SHA256 instance.

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