perf(ui): flush only the written descriptor range, not the whole heap (#61)

WriteBufferDescriptor / WriteSampledImageDescriptor / RegisterSampler /
WriteSwapchainDescriptors each FlushDevice()'d the entire multi-KB per-frame
descriptor heap after writing one few-byte descriptor. Add a ranged
VulkanBuffer::FlushDevice(offset, bytes) that flushes only the touched bytes,
rounding the range outward to nonCoherentAtomSize as vkFlushMappedMemoryRanges
requires (the WHOLE_SIZE path sidestepped that). The coherent-memory gate from
issue #60 is preserved — coherent memory still skips the flush entirely.

The descriptor writers now self-flush their written range, so the redundant
whole-heap flushes in UIRenderer::Initialize and the resize callback are gone.

Rounding lives in AlignMappedFlushRange (pure math) and is unit-tested without
a device in the new VulkanBufferRangedFlush test; the change was also verified
end-to-end by running HelloUI on a real GPU with validation layers (font-atlas
glyphs, sampler and storage-image descriptors all flush correctly, no VUIDs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

implementations/Crafter.Graphics-Device.cpp

@@ -604,6 +604,10 @@ void Device::Initialize() {
     };
     vkGetPhysicalDeviceProperties2(physDevice, &properties2);
 
+    // Cache the flush/invalidate alignment for ranged host-memory flushes
+    // (VulkanBuffer::FlushDevice(offset,bytes)).
+    nonCoherentAtomSize = properties2.properties.limits.nonCoherentAtomSize;
+
     // NVIDIA's brand-new VK_EXT_descriptor_heap acceleration-structure read
     // path faults (see #7); enable the SPIR-V rewrite workaround there. Other
     // drivers (and any future fixed NVIDIA driver, once this gate is removed)