Add BRANCH_DIR/BRANCH_MAG/BRANCH_ORD/BRANCH_URG/NUM_BRANCHES to
state_layout.cuh. Migrate the 4 clear literal branch-index accesses
in branch_grad_balance_kernel.cu (branch_norms_dev[0..3] in
grad_balance_isv_update).
Other branch-keyed arrays (branch_starts, branch_lens, branch_norms,
branch_scales) use the runtime `branch = blockIdx.y` variable or loop
counter — no raw literal index, so no migration needed. Rust call sites
use struct fields (branch_0_size etc.) or loop vars — not literals.
No behavioural change; pure refactor.
Plan 1 Task 4D. Spec §3 Invariant 8.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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