Add contract fuzz targets

[SlaVKsVolks]

Summary

• add a cargo-fuzz package under contracts/sorosave/fuzz
• add fuzz targets for create_group, contribute, and distribute_payout
• seed boundary and flow corpora for deterministic starting coverage
• build the contract as both cdylib and rlib so the fuzz harness can link it

Closes #36

Verification

• cargo fmt --all
• cargo check --manifest-path contracts/sorosave/fuzz/Cargo.toml --bins
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo build
• cargo clippy --manifest-path contracts/sorosave/fuzz/Cargo.toml --bins -- -D warnings

Note: cargo-fuzz is not installed in this local environment, so I verified the fuzz binaries through cargo check/clippy on their manifest.