feat: post-quantum signature support (SLH-DSA / FIPS 205)

[avrabe]

Context

wasmsign2 uses Ed25519 for Wasm module signatures. portal-co/wasmsign3 is an experiment replacing this with SLH-DSA (FIPS 205, stateless hash-based signatures via Shake128s) — a post-quantum scheme.

for safety-critical and long-lived embedded targets, post-quantum readiness may become a compliance requirement. rather than a separate tool, sigil could support SLH-DSA as an additional signing backend alongside Ed25519 and Sigstore keyless.

Scope

• evaluate SLH-DSA parameter sets (Shake128s, Shake256s, etc.) for signature size vs. security tradeoffs
• add SLH-DSA as a signing/verification backend in wsc-lib
• ensure no_std compatibility (the slh-dsa crate supports default-features = false)
• maintain backwards compatibility with existing Ed25519 signatures

References

• FIPS 205 — SLH-DSA
• slh-dsa crate
• portal-co/wasmsign3 — experimental SLH-DSA implementation for Wasm signatures