fix: add platform markers to GPU extras, use --extra cpu in CI, simplify lockfile workflow

- rocm/intel/gpu extras are x86_64-only; aarch64 wheels don't exist so uv
  failed to resolve them when required-environments includes aarch64
- test.yml: switch from --all-extras (broken by conflicts + missing wheels)
  to --extra cpu which is cross-platform and sufficient for unit tests
- update-lockfile.yml: drop old file-swap loop; single pyproject means a
  single uv lock run and a single uv.lock to commit

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
         run: uv python install 3.13
 
       - name: Install dependencies
-        run: uv sync --all-extras
+        run: uv sync --extra cpu
 
       - name: Run tests
         run: uv run pytest