Fix issues 1523 1519

[dorismaduegbunam]

This PR addresses two separate issues by expanding our test coverage and internal documentation on KYC gating limits.

Changes:

1. compute_expected_return Proptests:
   • Introduced compute_expected_return to quicklendx-contracts/src/profits.rs.
   • Added exhaustive property testing in src/test_compute_yield_props.rs ensuring the expected return remains non-negative, stays bounded by the system's MAX_RATE_BPS, and appropriately factors in the principal amount.

Closes #1523

2. KYC Gating Documentation:
   • Authored docs/KYC.md targeting operators, which breaks down the gating criteria between Business KYC (invoice listing, funding) and Investor KYC (bidding, withdrawals) using concrete JSON payload examples.
   • Linked the new guide in the top-level README.md for proper discoverability.

Closes #1519

Verification:

• Executed cargo test -p quicklendx-contracts --features fuzz-tests to successfully validate new properties locally.
• Maintained no_std compliance and verified WASM compilation via cargo build --target wasm32-unknown-unknown --release.
• Ensured formatting passes standard checks.

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[Baskarayelu]

Fix issues 1523 1519 — exactly what we wanted. merging 👍