Add Hardware Abstraction Layer (#56)

backend/docs/backend_safety_contract.md

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+Implementors must uphold all of the following for **every** call:
+
+* **Memory domains**: Pointers produced by to_ref() / to_mut() must be valid
+  in the target execution domain for Self (e.g., CPU host memory for CPU,
+  device memory for a specific GPU). If host↔device transfers are required,
+  perform them inside the implementation; do not assume the caller synchronized.
+
+* **Alignment & layout**: All data must match the layout, stride, and element
+  size expected by the kernel. size(), rows(), cols_in(), cols_out(),
+  n(), etc... must be interpreted identically to the reference CPU implementation.
+
+* **Scratch lifetime**: Any scratch obtained from scratch.tmp_slice(...) (or a
+  backend-specific variant) must remain valid for the duration of the call; it
+  may be reused by the caller afterwards. Do not retain pointers past return.
+
+* **Synchronization**: The call must appear **logically synchronous** to the
+  caller. If you enqueue asynchronous work (e.g., CUDA streams), you must
+  ensure completion before returning or clearly document and implement a
+  synchronization contract used by all backends consistently.
+
+* **Aliasing & overlaps**: If res, a, b, etc... alias or overlap in ways
+  that violate your kernel’s requirements, you must either handle safely or reject
+  with a defined error path (e.g., debug assert). Never trigger UB.
+
+* **Numerical contract**: For modular/integer arithmetic, results must be
+  bit-exact to the specification. For floating-point, any permitted tolerance
+  must be documented and consistent with the crate’s guarantees.