Add tests for the protocol-fee floor-division rounding in calculate_protocol_fee

[Baskarayelu]

Closes #654

Summary

• Add unit tests that pin calculate_protocol_fee floor-division rounding, zero-fee short-circuit, and overflow guard
• Test zero fee_bps short-circuit returns 0 without multiplying
• Test sub-threshold amounts (product < 10_000) yield fee of 0
• Test indivisible products (e.g. 3_333 × 1_000 bps) floor down correctly (333, not 334)
• Test representative 250-bps case on round amounts yields exact expected fee
• Test overflow guard: amount near i128::MAX at non-zero bps panics with PotentialOverflow (error 28. Arbitrator role management #28)
• Test security invariant: fee never exceeds gross amount, net payout never negative
• Integration test: three milestone releases at 10

[mikewheeleer]

love this — in it goes 👍