Add ComputeEncoder::batchBuildAS() and shared AS-build orchestration helper

[MarijnS95]

Depends on #1232
For #1158

Move acceleration-structure build commands behind the abstract ComputeEncoder interface so the orchestration (data upload, build-request creation, AS allocation, build recording) can live in one place rather than splitting across three backends.

ComputeEncoder gains a single batchBuildAS(ArrayRef<ASBuildItem>) method. Each item carries an AccelerationStructure plus a BLAS or TLAS build request via PointerUnion. The caller guarantees no inter-item memory dependencies inside a batch — backends record the whole batch with one barrier slot, no per-element barriers.

• Vulkan: single vkCmdBuildAccelerationStructuresKHR call covering the whole batch. TLAS items serialize VkAccelerationStructureInstanceKHR into a device-address upload buffer; BLAS items pull addresses from each VulkanBuffer (new getDeviceAddress accessor). Storage buffers transparently gain SHADER_DEVICE_ADDRESS + ACCEL_BUILD_INPUT_READ_ONLY flags when ray tracing is supported, with the matching VkMemoryAllocateFlagsInfo chained on every allocation.
• DX12: loop calling BuildRaytracingAccelerationStructure per item with no intermediate barriers; D3D12_RAYTRACING_INSTANCE_DESC is bit-identical to the Vulkan instance struct.
• Metal: lazy transition to MTL::AccelerationStructureCommandEncoder, deduplicates BLAS handles into the MTL::InstanceAccelerationStructureDescriptor's instancedAccelerationStructures array (Metal references BLASes by index, not GPU address).

A shared helper buildPipelineAccelerationStructures in lib/API/Device.cpp walks Pipeline::AccelStructs, uploads vertex/index data via the new createBufferWithData, builds requests, allocates AS objects, and issues two batchBuildAS calls (BLAS batch then TLAS batch — VUID-03403 forbids referencing a sibling dstAccelerationStructure in one command). Each backend's executeProgram calls this helper to build the pipeline's AS objects.

Descriptor binding for AS resources lands in the next PR — the tests progress past AS-build but still need the descriptor-write step.