Add LF magnitude-window integration utilities

[nikosarcevic]

This PR adds reusable LF magnitude-window integration utilities and moves repeated magnitude-grid integration logic into shared helpers.

Changes

• Added a generic variable-bound integrator in lfkit.utils.integrators.
• Added safe_divide for broadcast-safe ratios with invalid-denominator handling.
• Refactored LF photometry integrals to use the shared integration utilities.
• Added support for finite absolute-magnitude windows and apparent-magnitude windows converted using luminosity-distance callables.
• Kept optional K/E correction support for apparent-to-absolute magnitude conversion.
• Updated unit tests for broadcasting, scalar/array inputs, empty integration ranges, invalid bounds, non-finite LF/weight outputs, and safe division.

Motivation

This provides the low-level LFKit building block for magnitude-limited number-density calculations, while leaving science-specific weighting choices to downstream anlysis layers