OIS payer and receiver selection

MakeOis::with_type adds a Rust-core builder option for the fixed-leg side.

Type	Fixed leg	Overnight leg
SwapType::Payer	Pay	Receive
SwapType::Receiver	Receive	Pay

The existing default remains Payer. Repeated calls use the last value.

Usage

use libitofin::instruments::SwapType;

let receiver = builder.with_type(SwapType::Receiver).build()?;

Here builder is an existing MakeOis with its index, settings and optional fixed rate. Use fixed_vs_floating().swap_type() to inspect the chosen side.

Pricing contract

• The setter changes only the side: schedules, notional, day counts, overnight spread, fixing behavior and discount-curve selection are unchanged.
• For identical inputs, reversing the side reverses total and each leg's NPV and signed BPS. Fair fixed rate and fair overnight spread remain the same.
• With fixed_rate = None, the temporary fair-rate swap also uses the chosen side, and the final swap is built at that fair rate.
• Both sides retain the existing engine error propagation. Empty curves and missing fixings are not silently accepted for valuation.
• Handles and settings remain shared, preserving ordinary repricing on notifications, including curve relinks and evaluation-date updates.

Validation and scope

Whole-instrument fixtures use independent native QuantLib 1.43 OIS valuations on distinct nonflat forecast/discount curves. Tests cover both sides, fair-rate fill, EOM/stubs, payment lag, PV/BPS signs, fair-rate/spread rebuilds, default compatibility, repeated setter calls, relinks and empty-curve recovery.

This is additive Rust-core scope. It does not add receive_fixed, per-leg schedule overrides, a custom-engine builder option, telescopic/observation features, arithmetic averaging, or Python/C/Go bindings. Existing guarded unsupported options remain guarded; no release or version change is required.