Remove HashLib IHash leak from public IDigest#126
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IDigest is an adapter over HashLib4Pascal's IHash, but it re-exposed that IHash on its public contract via GetUnderlyingHasher/UnderlyingHasher. That baked HashLib into the public digest API (ClpIDigest.pas itself used HlpIHash) and advertised a capability not every digest has - TPrehash returned nil, so building an HMAC from a prehash digest passed nil into HashLib and crashed cryptically. Move the raw-hash access off IDigest onto a small internal capability interface, IBackingHashProvider (property BackingHash), implemented by TDigest. The public IDigest is now HashLib-free. - New ClpIBackingHashProvider.pas; IDigest drops GetUnderlyingHasher and its HlpIHash dependency. - TDigest implements IBackingHashProvider (GetUnderlyingHasher -> GetBackingHash); TPrehash's override still returns nil. - THMac and TPkcs5S2ParametersGenerator obtain the hash via Supports(digest, IBackingHashProvider, p) then p.BackingHash, raising a clear "digest does not provide a backing hash" error when it is nil/absent (fixes the prehash nil footgun). - TDsaParametersGenerator replaces the `hasher is TSHA1` type check with AlgorithmName = 'SHA-1', dropping the HlpSHA1 import.
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IDigest is an adapter over HashLib4Pascal's IHash, but it re-exposed that IHash on its public contract via GetUnderlyingHasher/UnderlyingHasher. That baked HashLib into the public digest API (ClpIDigest.pas itself used HlpIHash) and advertised a capability not every digest has - TPrehash returned nil, so building an HMAC from a prehash digest passed nil into HashLib and crashed cryptically.
Move the raw-hash access off IDigest onto a small internal capability interface, IBackingHashProvider (property BackingHash), implemented by TDigest. The public IDigest is now HashLib-free.
hasher is TSHA1type check with AlgorithmName = 'SHA-1', dropping the HlpSHA1 import.