SLH-DSA Support

### Problem:

AWS-LC supports ML-KEM (FIPS 203) and ML-DSA (FIPS 204) but not SLH-DSA (FIPS 205), the third of the three NIST-standardized post-quantum algorithms.

This issue is a general question to find out whether there are existing plans to support SLH-DSA in AWS-LC.

### Solution:

* **Does this change any public APIs?** Yes - it adds at least new key types and parameter-set
* **Which algorithm(s) will this impact?** SLH-DSA

FIPS 205 defines 12 potential variants, the cross product of the following choices:

- Hash function: SHA2 / SHAKE
- Security Level: 128 / 192 / 256
- Variants: s (small) / f (fast)

I don't think AWS-LC needs to support _all_ of them, but I also don't know how to select the ones to support.

### Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

* **RFC links:**:
* https://csrc.nist.gov/pubs/fips/205/final
* **Related Issues:** None found
* **Will the Usage Guide or other documentation need to be updated?** Yes! At least the supported-algorithms documentation.
* **Testing:**
* Validation against the NIST ACVP?
* **Will this change trigger AWS LibCrypto Formal Verification changes?** Yes if new algorithms are formally proven.
* **Should this change be fuzz tested?** Yes.

### Out of scope:

Is there anything the solution will intentionally NOT address?

_None_

Start with the linked FIPS 205 specification and compare the existing ML-KEM and ML-DSA support in AWS-LC. Define which SLH-DSA parameter sets and public APIs are in scope, then account for supported-algorithms documentation, NIST ACVP validation, fuzz testing, and any formal-verification impact.

Written by the indexing model from the issue text.