test: add contract fuzz targets

[TUPM96]

/claim #36

Summary

• add a cargo-fuzz package under contracts/sorosave/fuzz
• add fuzz targets for create_group, contribute, and distribute_payout
• add a shared Soroban harness with mocked auth, test token setup, randomized members, and randomized action ordering
• document how to run the targets and current findings
• keep the fuzz package excluded from the default workspace so normal contract CI is not forced to build libFuzzer dependencies

Validation

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

Note: cargo-fuzz is not installed in this local environment, so I validated that all fuzz bins compile via cargo check.

Payout details can be provided privately on acceptance.

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