Change signature scheme to Poseidon2

[kamilsa]

Signature Scheme Upgrade:

• Switched the signature scheme from Winternitz (SHA-3/SHAKE, 2^18 lifetime) to Target Sum encoding (Poseidon2 hash, 2^32 lifetime)

[Copilot]

Pull Request Overview

This PR upgrades the signature scheme from Winternitz encoding with SHA-3/SHAKE (2^18 lifetime) to Target Sum encoding with Poseidon2 hash (2^32 lifetime), significantly extending the key lifetime and switching to a ZK-friendly hash function.

• Updated signature scheme implementation to use SIGTopLevelTargetSumLifetime32Dim64Base8
• Extended lifetime from 262,144 epochs to 4,294,967,296 epochs
• Bumped package version from 0.1.0 to 0.2.0

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/c_hash_sig/src/lib.rs	Updated import and type alias to new Poseidon2-based Target Sum signature scheme; updated test to verify new 2^32 lifetime
crates/c_hash_sig/README.md	Replaced Winternitz scheme documentation with comprehensive Target Sum encoding parameters including hash function, dimensions, and lifetime details
crates/c_hash_sig/Cargo.toml	Bumped version from 0.1.0 to 0.2.0 to reflect the breaking API change

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