ADR 0004: backend boundary

Status: accepted

CPU NumPy/SciPy is the reference implementation. ArrayBackend owns array creation, elementwise operations, reductions, centered FFTs, interpolation, and optical blur hooks. Sensor engines receive a backend instance and do not call NumPy allocation/FFT/reduction functions directly; an AST guard and CPU injection-parity tests protect that rule.

File readers, configuration parsing, source quadrature, and metadata are explicit host-side boundaries. ArrayBackend.scalar is used only where a scalar must cross into geometry/metadata, and ArrayBackend.to_host remains the named escape hatch for file-facing workflows.

Amendment (post device-resident getframes gate): schema v1 now accepts numerics.device = "gpu". The detector adapter passes CuPy photon-rate arrays directly to getframes.Camera(..., device="gpu"); Frame.data and truth remain on device. The old private backend-injection hook remains for compatibility, but the configuration field is the supported user contract. CPU NumPy/SciPy remains the numerical reference.