holosoma-retargeting: pyproject-only + uv_build backend, fix README packaging

[clayrosenthal]

Converts holosoma-retargeting to a single declarative pyproject.toml with the uv_build backend — matching the same treatment applied to holosoma-inference and holosoma core.

Changes

• Moved README.md up one dir (holosoma_retargeting/README.md → package root). readme = "README.md" resolves relative to the project root, so the old location meant the README was never packaged — the long description was silently empty. It now embeds as text/markdown.
• pyproject.toml: build backend setuptools → uv_build (flat layout, module-root = ""); added Amazon FAR Team author, Apache-2.0 license, keywords, and trove classifiers. Dependencies and the dev extra are unchanged.
• Deleted setup.py — it was a redundant duplicate of the pyproject metadata.
• Added source-include data-file globs so the bundled robot descriptions, meshes, motion data, and templates ship in the wheel (urdf/xml/STL/obj/dae/mtl, npy/pt/pkl, the urdf jinja template, in-module docs). The old setuptools build had no MANIFEST.in / package_data, so it shipped none of these — a pip install got the code but not the assets.

Verification

• uv build → clean sdist + py3-none-any wheel; twine check PASSED on both.
• Single top-level holosoma_retargeting, 5 subpackages (matches the old find_packages() exactly).
• README embedded (12 KB); all data files land in the wheel (177 STL, 139 obj, 49 dae, 22 mtl, 21 xml, 10 urdf, 5 npy, 2 pt, 2 md, 1 pkl, 1 jinja).