Poseidon hasher integration

[chetanyb]

Summary

Integrates Poseidon2 (KoalaBear, width‑24) as an optional SSZ Merkle hasher via the hash-zig dependency, with a SHA256-compatible API wrapper specialized for 64-byte SSZ compressions.

Features

• SHA256-like interface (init / update / final) that enforces exactly 64 bytes per digest (no padding, no streaming).
• Poseidon2-24 KoalaBear backend pulled from hash-zig.
• 24-bit packing (22 limbs) → one Poseidon2 permutation → 32-byte output; no feed-forward.
• Zero hashes are recomputed with the active hasher; SSZ semantics preserved (single-chunk leaves returned directly).
• Backward-compatible default: SHA256; enable Poseidon with -Dposeidon=true.
• Added Plonky3 parity test suite runs only in Poseidon mode (checks against results from exact operations using Plonky3).

Important Notes

• SSZ-specific: Designed solely for SSZ merkleization/mix-in paths that always provide 64 bytes. final asserts buffer_len == 64.
• Not general-purpose: No padding or variable-length support; using it outside SSZ would be unsafe.
• Width guard: Compile-time checks ensure the Poseidon2 type is width-24 and exposes the required Field and permutation.
• Injectivity: 24-bit limbs (< p) avoid the lossy modulo reduction issue present in the old version.

Build Examples

# Default (SHA256)
zig build

# Poseidon2 (KoalaBear, width-24)
zig build -Dposeidon=true

[gballet]

Basically what we discussed during the team meeting

[chetanyb]

Closing in favour of #57 + blockblaz/hash-zig#97