Fix packet scatter to raw pointers

[jeongseok-meta]

Motivation

dr::scatter() accepts raw-pointer targets, but packet values currently try to call scatter_packet_ on the pointer type. Pointer expressions such as .data() also cannot bind to the existing lvalue target parameter. This prevents Momentum's rasterizer from building.

conda-forge currently carries a patch for this behavior. It can be removed after this fix is included in a Dr.Jit release.

Changes

• Preserve the existing lvalue overload and add a constrained overload for pointer expressions.
• Dispatch raw-pointer packet scatters through the outer static array, selecting the generic element-wise fallback; JIT/AD specializations require an array target.
• Normalize scalar and SIMD indices to unsigned 32-bit values without changing their nesting depth, so each recursive packet layer contributes its flattening stride.
• Require a static native outer packet with one clear compile-time diagnostic, without instantiating pointer-incompatible overloads, while preserving native nested DynamicArray recursion.
• Cover padded, nested, and deeply nested packet layouts, native nested DynamicArray values, signed indices, and explicit-template lvalue calls.

Testing

• Fresh GCC 15/CMake build: the extension compiled and the four focused raw-pointer packet tests passed, covering scalar, nested, deeply nested, and native nested DynamicArray values.
• Standalone scalar, masked, nested, and signed-index packet scatter probes passed.
• A depth-three packet value with a depth-one SIMD index produces [1, ..., 16], confirming recursive flattening without collisions.
• Native nested DynamicArray packet scatter produces [1, 2, 3, 4, 5, 6]; dynamically sized outer values and JIT/AD packet values fail with only the intended unsupported-value diagnostic.
• The previous vendored broadcast fallback also supports native nested DynamicArray values but fails to compile for static arrays containing JIT/AD elements; Momentum's raw-pointer callers use native fixed-size packets.
• tests/test_call_ext.py: 128 passed
• Unpatched Dr.Jit reproduces Momentum rasterizer failures at both pointer-expression and packet-dispatch call sites.
• With this change, Momentum's rasterizer and pymomentum renderer build, and pymomentum/test/test_renderer.py passes all 8 tests.
• python -m py_compile tests/test_memop_ext.py
• git diff --check