Feature/gaussian1d float float swaption coverage

[losterbr]

Summary

Improve coverage and robustness of Gaussian1dFloatFloatSwaptionEngine.

Changes

• Added comprehensive Gaussian 1D float-float swaption coverage, including:
  • Payer and receiver swaptions
  • Probability and payoff extrapolation modes
  • Rebates and OAS
  • Principal exchanges and caps/floors
  • CMS and swap-spread indices
  • Cash-settlement validation
• Added calibration boundary checks for:
  • No remaining reset dates
  • Zero remaining notionals
• Added deterministic regression checks for rollback values, probability results, and OAS discounting.
• Improved rollback performance by:
  • Removing unused event state.
  • Reusing probability interpolators across state-grid points.
  • Computing the rollback OAS discount factor once per event.
  • Removing an unused FloatFloatSwap copy.
• Preserved behavior across the optimized and baseline implementations.

[coveralls]

coverage: 78.607% (+0.4%) from 78.251% — losterbr:feature/gaussian1d-float-float-swaption-coverage into lballabio:master