Contracts: Add fuzz target for campaign contract registration logic

[joelpeace48-cell]

Summary

The rewards contract has a fuzz target at contracts/rewards/fuzz/fuzz_targets/fuzz_balance.rs that randomizes credit/claim sequences. The campaign contract has no fuzz coverage, leaving participant registration, Merkle proof verification, cap enforcement, and time window logic untested by property-based randomization.

Problem

Campaign contract logic involves multiple interacting invariants:

• Registration is idempotent (double register returns false, count does not increase)
• Cap is never exceeded (participant_count ≤ max_cap when max_cap > 0)
• Time window is enforced (no registration outside [start, end])
• Merkle proof rejection (invalid proof must never register participant)

These invariants can be broken by edge cases that unit tests miss. Fuzzing catches them systematically.

Acceptance Criteria

• Create contracts/campaign/fuzz/ directory structure mirroring contracts/rewards/fuzz/
• Add fuzz_targets/fuzz_register.rs that:
  • Generates random (participant, cap, start, end, current_ledger, merkle_root_set) inputs
  • Calls register with valid/invalid parameters
  • Asserts: participant_count <= max_cap always
  • Asserts: if participant was successfully registered twice, count only incremented once
  • Asserts: if outside time window, Error::OutsideTimeWindow is returned
• Add Cargo.toml and Cargo.lock for the fuzz workspace
• Document how to run the fuzz target in contracts/campaign/README.md or a new FUZZING.md
• Wire the fuzz build into contracts-ci.yml as an optional cargo fuzz build step

References

• contracts/rewards/fuzz/fuzz_targets/fuzz_balance.rs — existing fuzz target to mirror
• contracts/campaign/src/lib.rs — target logic
• .github/workflows/contracts-ci.yml

[GreatShinro]

@GreatShinro has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi, can I work on this please?

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[dikachim060-wq]

@dikachim060-wq has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

can i work on this?

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