test: fuzz weighted distribute

[pimaster900]

test: fuzz weighted distribute

Adds a cargo-fuzz target for batch_distribute in the revenue pool contract.

What's new

• contracts/revenue_pool/fuzz/Cargo.toml — fuzz crate wired to libfuzzer-sys
• contracts/revenue_pool/fuzz/targets/weighted_distribute.rs — fuzz target that decodes raw
  bytes into weighted payment legs and asserts three invariants on every execution:
  1. Conservation — on success, pool balance decreases by exactly the sum of distributed
     amounts
  2. Rejection invariant — on any invalid input (zero amount, duplicate recipient, oversized
     batch, insufficient balance), balance is unchanged
  3. No uncontrolled panic — unexpected aborts surface as fuzzer failures

Encoding

Input bytes are parsed as 3-byte legs: 1 byte selects a developer from a fixed pool of 8
addresses (naturally produces duplicate-recipient cases), 2 bytes encode the u16 amount
(naturally produces zero-amount cases). No manual corpus seeding required.

Running

cargo install cargo-fuzz
cd contracts/revenue_pool
cargo fuzz run weighted_distribute

Workspace impact

Fuzz crate added to workspace.members but not default-members — cargo build and cargo test at
the root are unaffected.

closes #540

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