mlkem: export the seed instead of the raw private key

[qmuntal]

Upstream uses the MLKEM seeds instead of raw private keys to pass keys around. We should do the same. Added a test to ensure we don't regress.

[Copilot]

Pull Request Overview

This PR changes the ML-KEM implementation to export and use 64-byte seeds instead of the full raw private key material (2400 bytes for MLKEM768, 3168 bytes for MLKEM1024), aligning with the upstream standard library approach.

Key changes:

• Updated to use MLKEM_PRIVATE_SEED_BLOB instead of MLKEM_PRIVATE_BLOB for exporting/importing keys
• Changed DecapsulationKeyMLKEM768 and DecapsulationKeyMLKEM1024 types to use 64-byte seeds instead of full key material
• Added test assertions to verify that decapsulation key sizes match between CNG and stdlib implementations

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
internal/bcrypt/bcrypt_windows.go	Added MLKEM_PRIVATE_SEED_BLOB constant and MLKEM_PRIVATE_SEED_MAGIC magic number, with formatting alignment improvements
cng/mlkem.go	Removed full key size constants, updated functions to use seeds, and changed type definitions to use 64-byte seeds instead of full key material
cng/mlkem_test.go	Changed import from math/rand to crypto/rand for proper cryptographic randomness, and added size validation checks for interoperability testing

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