Implement bitboard square mapping and position representation

[binarylab2022-del]

Objective

Create the initial bitboard foundation for WinDama by representing the 32
playable squares of the 8×8 board with bits.

The existing readable board representation remains the reference
implementation. The new bitboard representation will later support optimized
move generation, equivalence testing, search improvements, and FPGA-oriented
research.

Required work

1. Define the playable-square mapping

Create a deterministic mapping between:

Board coordinate (row, column)
        ↕
Playable-square index 0..31
        ↕
Single-bit mask