Legal-only move generation using the check-mask + pin-mask technique.

Instead of generating pseudo-legal moves and filtering them, legal_moves_int/4 computes the checking pieces (check_mask) and the pinned rays (pin_rays) once per position, then emits only legal moves: king moves avoid the danger set, and all other moves are masked by check_mask and their pin ray. Double check exits early with king moves only.

Moves are emitted as packed integers (see Echecs.Move) and converted to structs only at the API boundary by legal_moves/1. The captures/1 and quiets/1 variants share the same pipeline for search use.