Add differential test harness comparing pre/post-refactor outputs for profit and fee math

[Baskarayelu]

Description

profits.rs::calculate_profit, calculate_treasury_split and fees.rs::calculate_transaction_fees have been refactored repeatedly (tiered fees, dust handling, treasury splits). To catch silent semantic drift during future refactors, add a differential harness that captures a frozen golden-vector corpus of (investment_amount, payment_amount, fee_bps, tier, volume) -> (investor_return, platform_fee, treasury, dust) and re-evaluates it against the live implementation. Any divergence fails CI and forces the author to explicitly bless the change.

Requirements and context

• Secure: bless workflow is admin-reviewed via PR; no auto-bless.
• Tested: corpus covers at least 500 input combinations including edge cases at i128::MAX, zero payment, overpayment.
• Documented: docs/profit-fee-differential.md describing corpus refresh policy.
• Reference: src/profits.rs, src/fees.rs.

Suggested execution

git checkout -b feature/profit-fee-differential

• Add tests/profit_fee_golden.rs; store corpus as JSON under tests/fixtures/profit_fee_corpus.json.
• Implement scripts/refresh-profit-fee-corpus.sh that regenerates the corpus from the current implementation, gated behind an env var to prevent accidental blessing.
• Rust doc comments on the corpus loader explaining bless semantics.
• Docs: docs/profit-fee-differential.md.
• Validate that verify_no_dust is exercised across the corpus.

Test and commit

Run cargo test --test profit_fee_golden and confirm zero diffs against the committed corpus. Include the output and a note that any future delta requires a corpus refresh PR.

Example commit message

test(profits): add differential golden-vector harness for profit and fee math

Guidelines

• Minimum 95% test coverage
• Clear documentation in docs/profit-fee-differential.md
• Timeframe: 96 hours