v0.5.1 — backend/build fixes (--gpu on Vulkan, HIP decimation arch, Windows packaging)

v0.5.1 — backend/build fixes

Patch release fixing reported crashes/errors on specific build & backend combinations.

Fixes

• Honor --gpu on Vulkan (#16). --gpu N was applied only on the CUDA path;
  the Vulkan/generic path always picked the largest-VRAM device. It now selects the
  N-th GPU/IGPU device (matching CUDA's index semantics); --gpu 0 keeps the
  largest-VRAM default, and an out-of-range index warns and falls back.
• HIP decimation kernel built for the wrong GPU arch (#14). An auto-detected
  CMAKE_HIP_ARCHITECTURES could override a user's -DGPU_TARGETS, so the custom
  QEM decimation kernel launch failed with "device kernel image is invalid" and
  decimation silently fell back to the slow CPU path. GPU_TARGETS /
  AMDGPU_TARGETS are now authoritative for the kernel's arch, and the CPU
  fallback logs a clear, non-alarming note.
• Windows app packaging. The desktop-app CI job and install.ps1 now use the
  NSIS setup .exe (per-user, silent-installable) that Tauri actually builds,
  fixing the Windows Studio-app publish step.

Install / usage unchanged — see docs/getting-started.md.

Server bundles (trellis-<backend>-<os>-x64) and the app installers
(trellis-studio-*) are attached below once the release CI finishes building.