Improve Test Coverage

[nloding]

Problem

Test coverage has several gaps that should be addressed.

Untested PGN Scenarios

• Deeply nested variations (5+ levels)
• Games with hundreds of NAGs
• Very long comments (multi-line, special characters)
• Malformed PGN (unclosed brackets, invalid moves)
• Empty games (no moves)

Untested Database Scenarios

• Maximum database size (16M games)
• Concurrent access patterns
• Database recovery after crash
• Very large games approaching size limit

Untested Encoding Scenarios

• En passant capture encoding
• All castling scenarios (both colors, both sides)
• Games with 255+ half-moves (truncation in index)
• Underpromotion edge cases

Priority

Medium - improves reliability