Add Hagan-West monotone-convex discount interpolation (#46)

[AIM-IT4]

Addresses a part of #46 (Hagan–West monotone-convex spline; other requested methods remain out of scope).

• Implements continuous instantaneous-forward interpolation using the piecewise Hagan–West 2008 regions and exact integrated forwards.
• Reproduces every supplied discount-factor knot by construction; supports mixed/negative forward rates and flat-forward extrapolation.
• Wires InterpTypes.MONOTONE_CONVEX through Interpolator.fit/interpolate and the existing DiscountCurve API, including callers whose first knot is later than t=0.
• Does not mark the method as bootstrap-local: interpolation can change when later knots are added.
• Tests pricing-knot/integral consistency (numerical quadrature), flat curves, negative rates, the published reference example and existing interpolation/discount-curve behaviors.

Local: 29 tests passed (new coverage and existing interpolation/discount tests).

Reference: Hagan and West, Methods for Constructing a Yield Curve (2008), equations 22–32 (https://dlu-umich.github.io/docs/HaganWest.pdf). CI pending.