Support irregular volatility surface inputs

[asancdec]

Replace core::VolSurface<T> with irregular volatility surface support

Motivation

The current core::VolSurface<T> assumes a rectangular grid:

• one shared strike/moneyness vector
• one maturity vector
• one Matrix<T> of vols where each tenor has the same number of strikes

This is convenient for model surfaces, but real market data is often irregular: each maturity/tenor may have a different set of listed strikes or log-moneyness points.

Goal

Replace the current rectangular core::VolSurface<T> representation with first-class support for irregular volatility surfaces where each maturity owns its own strike/moneyness/vol slice.

Possible shape:

template <std::floating_point T>
struct VolSlice
{
    T maturity;
    T forward;
    T discountFactor;
    Vector<T> strikes;
    Vector<T> moneyness;
    Vector<T> vol;
};

template <std::floating_point T>
class IrregularVolSurface
{
    Vector<VolSlice<T>> slices_;
};

Requirements

• Each maturity/tenor should own its own volatility slice.
• Each slice should allow a different number of strikes.
• Each slice should validate that strikes, moneyness, and vol have matching sizes.
• Maturities should be strictly increasing.
• Strikes/moneyness inside each slice should be ordered consistently.
• Existing code using core::VolSurface<T> should be migrated to the new irregular representation.
• Model pricing and calibration code should consume the new structure directly.

Migration scope

Replace usages of the current rectangular surface in:

• core surface construction
• Heston pricing inputs
• SVI/local-vol calibration inputs where applicable
• integration tests
• regression fixtures
• performance tests

Out of scope

• Arbitrage repair.
• Full volatility surface interpolation.
• Calibration algorithm redesign beyond the data-structure migration.
• Backward compatibility with the old rectangular VolSurface<T> unless explicitly needed during transition.

Acceptance criteria

• Add the new irregular volatility surface representation.
• Remove or deprecate the old rectangular core::VolSurface<T> API.
• Add unit tests for valid irregular surfaces.
• Add unit tests for variable strike counts across maturities.
• Add unit tests rejecting inconsistent slice dimensions.
• Add unit tests rejecting non-increasing maturities.
• Update existing model, integration, regression, and performance tests to use the new representation.
• Document the new surface layout and expected invariants.