test: Expanded contract fuzzing across all entrypoints

[joelpeace48-cell]

Background & current state

Property-based testing is tracked (#362) and some tests exist (contracts/*/src/test.rs), but fuzzing coverage isn't comprehensive across all state-changing entrypoints (credit/claim/vesting/tiers/redeem/register/admin ops).

Goal

Expand contract fuzzing (proptest/arbitrary) to every state-changing entrypoint, asserting invariants hold under random sequences.

Technical design

• For each contract, generate random valid/invalid call sequences and assert invariants (no underflow/overflow, claimed ≤ credited, reserve solvency, participant-count consistency, nonce monotonicity). Reuse the invariants from the formal-verification issue (NEW-053) as oracles.
• Seed corpus + CI fuzz job.

Edge cases

• Adversarial sequences (claim before credit, double-claim, replayed admin nonce).
• Boundary amounts (0, u64::MAX).
• Interleaved campaign/rewards interactions (integration crate).

Task breakdown

• Fuzz harnesses per entrypoint + invariants.
• Integration fuzz across contracts.
• CI fuzz job + seed corpus.

Acceptance criteria

• Fuzz suite covers all state-changing entrypoints and runs in CI; invariants hold.

Testing & verification

• CI fuzz job; intentionally-broken invariant must fail.

Out of scope

• Backend API fuzzing (NEW-062).

Dependencies / related

• Extends Testing: Add contract property-based testing with arbitrary data generators #362; shares invariants with NEW-053.

Difficulty: medium · Effort: M