Pin detection (compute_pins)

🎯 Goal

Prevent pieces from exposing their king.

🧠 Key Idea

X-ray scanning from the king along sliding directions.

🛠️ Tasks

- Scan 8 directions from the king
- Detect:
  - first friendly piece
  - second enemy sliding piece
- Populate:
  - Bitboard pinned
  - Bitboard pin_ray[64]

✅ Acceptance Criteria

- Handles bishop, rook, pawn pins
- Pinned pieces are restricted correctly