Handle parallel rays on AABB faces without NaN intersections

[Afloat16]

A ray parallel to an AABB face produces 0/0 when its origin lies on that face. intersect_aabb then returns NaN crop bounds, including through Cameras.generate_rays; even interior parallel rays can have NaN backward gradients from the unused divisions.

Handle exact-zero directions using the slab constraint: a parallel ray inside the slab imposes no parameter bound; a parallel ray outside the slab misses the box. Protect the divisor before arithmetic and intersect the remaining finite intervals. Nonzero directions, including very small ones, retain the existing geometry and gradients.

Validation:

• 13 focused tests passed on CPU Torch 2.14.1; unchanged source: 9 failed, 4 passed.
• Tests cover float32/float64 and signed zero, box faces/edges, outside slabs, inside origins, analytic origin/direction/box gradients and gradcheck, small nonzero directions, rotated OBB crop boxes, actual camera ray generation, random nonparallel forward/gradient parity and empty batches.
• Existing AABB tests plus regressions: 15 passed. All existing utils/cameras tests plus regressions: 44 passed (existing deprecation warnings).
• Repository Ruff 0.12.2 configured lint/format checks for changed files, license check and git diff --check passed.

Full ns-dev-test, GPU/nerfacc execution and rendering/performance benchmarks were not run for this patch.