Document fixed/kink curves and expose curve params getter

[liobrasil]

Summary

• expand docs/interest_rate_and_protocol_fees.md so readers can understand debit and credit rate behavior without reading Cadence source
• add clear FixedCurve vs KinkCurve formulas and a concrete Aave-style volatile profile example (45% optimal utilization, 0% base, 4% slope1, 300% slope2)
• add public Pool read method getInterestCurveParams(tokenType) to expose curve configuration and current per-second rates
• add new script cadence/scripts/flow-alp/get_interest_curve_params.cdc for direct querying by token type

Validation

• flow test cadence/tests/update_interest_rate_test.cdc
• flow test cadence/tests/interest_curve_test.cdc
• flow test cadence/tests/pool_creation_workflow_test.cdc