Define DIR_SHORT=0, DIR_HOLD=1, DIR_LONG=2, DIR_FLAT=3, NUM_DIRECTIONS=4 in
state_layout.cuh. Migrate all literal dir_idx/raw_dir comparisons to named
constants across experience_kernels.cu (15 sites), trade_physics.cuh (3 sites),
and backtest_metrics_kernel.cu (2 sites). Raw integer comparisons (== 0/1/2/3)
eliminated from all direction-branch consumers.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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