Add support for payment lag in cash flow calculations. This is needed…

[novitk]

… for OIS(SOFR, ESTR, etc).

Ensured backward compatibility by defaulting paymentLag to 0.

QLNet

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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Tests
[ ] Other

Description

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for a payment lag on overnight indexed swaps by threading a paymentLag parameter through the fixed and overnight legs and the cashflow vector builder.

Changes:

• Adds paymentLag parameter (default 0) to OvernightIndexedSwap constructor and applies it to both legs.
• Adds withPaymentLag builder method on FixedRateLeg and OvernightLeg, with a helper calcPayDate on FixedRateLeg.
• Threads paymentLag through CashFlowVectors.OvernightLeg to adjust payment dates.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/QLNet/Instruments/OvernightIndexedSwap.cs	Adds optional paymentLag constructor parameter and forwards it to both legs.
src/QLNet/Cashflows/OvernightIndexedCoupon.cs	Adds withPaymentLag builder method on OvernightLeg and passes lag through to the cashflow builder.
src/QLNet/Cashflows/FixedRateCoupon.cs	Adds withPaymentLag and calcPayDate helper to apply lag in FixedRateLeg.value().
src/QLNet/Cashflows/Cashflowvectors.cs	Adds optional paymentLag parameter that shifts the computed payment date.

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