Add vectorized FixedRateBonds collection for bulk bond construction

[fumitoh]

Summary

This PR introduces a new FixedRateBonds class that enables efficient bulk construction and management of fixed-rate bonds using parallel arrays. This complements the existing singular FixedRateBond class and allows users to work with collections of bonds while maintaining individual bond reconstruction on demand.

Key Changes

• New FixedRateBonds Cython class (lifelib_pyql/portfolio/instruments/bonds.pyx):

  • Stores bond parameters as parallel numpy arrays (settlement days, face amounts, coupons, redemptions, issue dates)
  • Supports both 1D and 2D coupon arrays (with NaN-padding for variable coupon counts)
  • Implements __getitem__ for both integer indexing (returns singular FixedRateBond) and slicing (returns new FixedRateBonds)
  • Handles scalar and array redemptions with automatic broadcasting
  • Converts numpy datetime64[D] values to pyql Date objects on demand
  • Validates array dimensions and lengths at construction time

• Type definitions (lifelib_pyql/portfolio/instruments/bonds.pxd):

  • Cython extension type declarations for efficient array access

• API export (lifelib_pyql/portfolio/api.py):

  • Exports FixedRateBonds from the portfolio module for public use

• Comprehensive test suite (test/test_portfolio_bonds.py):

  • 20+ test cases covering construction, indexing, slicing, property access
  • Validation of consistency between indexed bonds and individually constructed FixedRateBond instances
  • Edge cases: negative indexing, out-of-bounds access, type validation
  • Support for optional issue dates and variable coupon structures

Notable Implementation Details

• Uses numpy datetime64[D] to pyql Date conversion with NaT handling
• Lazy bond reconstruction: individual bonds are created only when accessed via indexing
• Slice operations return new FixedRateBonds instances with properly sliced underlying arrays
• Supports optional issue dates (None is valid) for bonds without explicit issue date information
• Validates all input array lengths match the number of schedules at construction time

https://claude.ai/code/session_01JRwjJ46R8sg7EGof6X3ZWH