feat: add Board.AttacksFrom

[CosmicVeil]

Summary

Adds Board.AttacksFrom(Square) []Square to expose squares attacked by a
piece independently of legal-move restrictions.

Closes #70.

Behavior

• Includes friendly occupied squares as defended squares
• Sliding attacks include the first blocker and stop beyond it
• Pawn attacks include diagonals regardless of occupancy
• Ignores pins, check, and side to move
• Empty or invalid source squares return an empty result

Testing

• Added tests for every piece type
• Added pawn edge-file tests
• Added friendly-blocker and sliding-blocker tests
• Added pinned-piece tests
• Added benchmark coverage

Verification

• go test ./...
• go test -race ./...
• go test -bench=AttacksFrom -benchmem