## Cross-tensor op asymmetry Both ops consume two inputs (current plane + reference plane) on `forward` and two inputs (residual + reference plane) on `inverse`. The `Op` trait was designed for single-tensor chains; cross-tensor ops bend this by accepting the reference as the second element in the slice on both sides: - `forward(&[current, reference])` → `[residual]` - `inverse(&[residual, reference])` → `[original]` The runtime is responsible for supplying the reference as `inputs[1]` / `outputs[1]` respectively. This is documented here so chain authors know the convention. ## XorDelta Byte-level XOR residual. Because XOR is involutive, `forward` and `inverse` are the same operation: `residual[i] = a[i] ^ b[i]`. Round-trip is guaranteed byte-exact for all inputs. ## FloatDelta (DONE_WITH_CONCERNS) Element-wise float subtract (`output = current - reference`). Currently only `f32` is supported. The round-trip `inverse(forward(a, b), b)` computes `(a - b) + b`, which IEEE 754 does NOT guarantee to equal `a` bit-exactly when `a - b` introduces a rounding error. **This op is marked DONE_WITH_CONCERNS**: it violates the PPG byte-exact round-trip invariant for arbitrary f32 inputs where `|a - b|` causes sub-normal rounding. It exists for opportunistic delta encoding where near- identical tensors make the residual very small and coding gain outweighs the approximation. A future version may use integer delta on the raw bit patterns (which is lossless) instead.

**DONE_WITH_CONCERNS**: IEEE 754 does not guarantee that `(a - b) + b == a` bit-exactly for all inputs. This op may violate the PPG byte-exact round- trip invariant. Use `XorDelta` instead when lossless is required. Currently only `dtype_code` corresponding to `f32` is supported. **Input convention** (both forward and inverse): - `inputs[0]` / `outputs[0]`: the plane being encoded or the residual. - `inputs[1]` / `outputs[1]`: the reference plane.

`dep_idx` identifies which dependency tensor supplies the reference. **Input convention** (both forward and inverse): - `inputs[0]` / `outputs[0]`: the plane being encoded or the residual. - `inputs[1]` / `outputs[1]`: the reference plane (same shape). Both are consumed; one residual plane is produced.