[FEATURE] Support callable amortizing fixed-rate bonds (sinkable / sinking fund)

[amaggiulli]

Summary

QLNet supports callable fixed-rate bonds and amortizing fixed-rate bonds, but not a bond that is both callable and amortizing/sinkable.

This becomes a problem for municipal bonds with a sinking fund schedule, because a bullet callable model does not correctly represent:

• outstanding notional at each call date
• price/yield to call
• duration to call / duration to maturity
• effective duration / OAS duration

Problem

CallableFixedRateBond assumes a single notional / bullet redemption structure.

For sinkable callable bonds, principal declines over time according to a sinking fund schedule, so callable analytics should be based on the outstanding principal at each exercise date.

Today this requires downstream custom code.

Proposed enhancement

Add first-class support for callable amortizing fixed-rate bonds in QLNet.

Possible shape:

• a new CallableAmortizingFixedRateBond
• or an extension of the existing callable bond path to accept a notional schedule

The implementation should support:

• deterministic coupon cash flows on an amortizing notional schedule
• principal/redemption cash flows from the sink schedule
• call exercise amounts based on outstanding notional at each call date
• correct behavior for yieldToCalls, priceToCalls, yieldAt, priceAt, and durationAt
• compatibility with callable tree pricing (TreeCallableFixedRateBondEngine)

Acceptance criteria

• callable amortizing/sinkable fixed-rate bonds can be modeled directly in QLNet
• call analytics respect the sinking principal schedule
• tree pricing works with the amortizing callable structure
• regression tests cover both bullet callable and sinkable callable cases