Define MAG_QUARTER=0, MAG_HALF=1, MAG_FULL=2, NUM_MAGNITUDES=3 in
state_layout.cuh. Migrate the one unambiguous semantic comparison in
trade_physics.cuh (compute_target_position_4branch magnitude scaling).
Arithmetic operations on mag_idx (mag_idx±1, 2-mag_idx, mag_idx<2) are
runtime values, not semantic comparisons, and are not migrated.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>