Phase 1 of the rustytorch f32→f64 plan, backend layer. All 11 ops modules
(basic/creation/unary/gemm/reduction/activation/shape/conv/pooling/normalization/
attention) are now generic over the element via a `CpuFloat` bound
(`num_traits::Float + Send + Sync + 'static`); f32/Vec<f32> → E/Vec<E>, literals →
E::zero()/one()/from(..). The ops were already pure scalar + rayon (no SIMD), so
the f32 path is byte-identical (E inferred as f32 under CpuBackend) — no SIMD/BLAS
dual-path needed.
Adds `CpuBackendF64` (FloatElem = f64, TensorPrimitive = CpuTensorPrimitive<D,f64>)
delegating to the same generic ops, plus the DeviceOps<CpuBackendF64> impl.
CpuBackend (f32) untouched.
Validated: 35 tests pass (33 original f32 + 2 new f64); `cpu_backend_f64_exceeds_
f32_precision` preserves 1+2^-30 (f32 rounds to 1.0) — proves genuine f64. rtx-tensor
(dependent) still builds. clippy clean.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
