Add SLH-DSA

[mikelodder7]

This exposes the SLH-DSA signature methods and also adds the keypair_from_seed similar to how its been done for ml-dsa and falcon. @shemnon requested this signature type. These changes are needed for downstream bedrock

[greptile-apps]

Greptile Summary

Added keypair_from_seed functionality to all 12 SLH-DSA variants, enabling deterministic keypair generation from seed material.

Key changes:

• Exposed keypair_from_seed API in sig_slh_dsa.h for all variants (SHA2 and SHAKE, 128/192/256-bit, small/fast)
• Implemented functions consistently across all wrappers using slh_keygen_internal
• Added seed length validation requiring 3 * n bytes (SK.seed || SK.prf || PK.seed)
• Properly wired function pointers in *_new() constructors (previously NULL)
• Imported slh_param.h header for parameter access

Implementation pattern:
All implementations follow identical structure - validate seed length against 3 * prm->n, then call slh_keygen_internal with properly offset seed components. Matches existing patterns in ML-DSA and Falcon implementations.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The implementation is consistent across all 12 variants, follows established patterns from ML-DSA and Falcon, includes proper seed validation, and correctly uses the underlying slh_keygen_internal function. The changes are straightforward API additions with no modifications to existing logic.
• No files require special attention

Important Files Changed

Filename	Overview
src/sig/slh_dsa/sig_slh_dsa.h	Added keypair_from_seed function declarations for all 12 SLH-DSA variants
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_sha2_128s.c	Implemented keypair_from_seed with seed validation for SHA2-128s variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_sha2_128f.c	Implemented keypair_from_seed with seed validation for SHA2-128f variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_sha2_192s.c	Implemented keypair_from_seed with seed validation for SHA2-192s variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_sha2_192f.c	Implemented keypair_from_seed with seed validation for SHA2-192f variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_sha2_256s.c	Implemented keypair_from_seed with seed validation for SHA2-256s variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_sha2_256f.c	Implemented keypair_from_seed with seed validation for SHA2-256f variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_shake_128s.c	Implemented keypair_from_seed with seed validation for SHAKE-128s variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_shake_128f.c	Implemented keypair_from_seed with seed validation for SHAKE-128f variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_shake_192s.c	Implemented keypair_from_seed with seed validation for SHAKE-192s variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_shake_192f.c	Implemented keypair_from_seed with seed validation for SHAKE-192f variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_shake_256s.c	Implemented keypair_from_seed with seed validation for SHAKE-256s variant
src/sig/slh_dsa/wrappers/pure/slh_dsa_pure_shake_256f.c	Implemented keypair_from_seed with seed validation for SHAKE-256f variant

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[User calls keypair_from_seed] --> B{Validate seed_len}
    B -->|seed_len != 3*n| C[Return OQS_ERROR]
    B -->|seed_len == 3*n| D[Parse seed into components]
    D --> E[SK.seed = seed<br/>0 to n bytes]
    D --> F[SK.prf = seed + n<br/>n to 2n bytes]
    D --> G[PK.seed = seed + 2n<br/>2n to 3n bytes]
    E --> H[slh_keygen_internal]
    F --> H
    G --> H
    H --> I[Generate keypair deterministically]
    I --> J[Return OQS_SUCCESS]

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[greptile-apps]

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[manel1874]

What if we have variables for seed, seed + prm->n, seed + 2 * prm->n? Maybe its not so idiomatic but improves readability.