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

[codecov]

Codecov Report

❌ Patch coverage is 72.50000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.44%. Comparing base (2b08106) to head (899a883).

Files with missing lines	Patch %	Lines
attacks.go	72.50%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   80.51%   80.44%   -0.08%     
==========================================
  Files          27       28       +1     
  Lines        4424     4464      +40     
==========================================
+ Hits         3562     3591      +29     
- Misses        726      737      +11     
  Partials      136      136              

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