Perf: Avoid cmdlet re-entry for public-key derivation in ConvertFrom-SodiumSealedBox

[MariusStorhaug]

Summary

Remove internal cmdlet invocation overhead in ConvertFrom-SodiumSealedBox when -PublicKey is omitted.

Why

Current flow calls Get-SodiumPublicKey from within decrypt processing, which incurs PowerShell cmdlet pipeline/binding overhead in hot loops.

Proposed change

• In the decrypt path, derive public key directly via interop (crypto_scalarmult_base) instead of invoking another cmdlet.
• Keep output and validation semantics equivalent.
• Ensure private key material handling remains secure and zeroed where applicable.

Acceptance criteria

• Decrypt scenarios with and without -PublicKey remain functionally equivalent.
• Existing tests pass.
• Decrypt benchmark loop shows improved throughput when -PublicKey is not supplied.
• No regression in error messages for invalid key material.