fix(MeshHelper): compressed mesh bone weights and UV sets

[nichinichisou0609]

Meshes imported with Mesh Compression store their vertex data in m_CompressedMesh. Two values decoded from it are wrong; one commit each, and each commit adds a regression test that fails on its parent and passes with the fix.

• Implied fourth bone weight. A compressed mesh stores up to three 5-bit weights per vertex and implies the fourth when they do not add up to 31. Since MeshHandler: Fix weightsData dequantization error #338 the running sum counts 5-bit units, but the implied weight is still computed as 1 - sum, so weights 10, 10, 5 give a fourth weight of -24 and every skinned vertex with four influences gets a negative weight. It is now (31 - sum) / 31, the same formula as AssetStudio's DecompressCompressedMesh, and the four weights of a vertex add up to 1.
• Third and later UV sets. With m_UVInfo set, the UV sets are stored back to back in m_UV. The read offset was assigned the size of the current set instead of advancing by it, so every set after the second one was read from the position of the second set and returned a copy of it. The offset now accumulates, as in AssetStudio.

tests/test_compressed_mesh.py (new) decodes a synthetic CompressedMesh: one vertex whose fourth weight is implied and one with a single full weight, and three UV sets with 2, 3 and 2 components in channels 0, 1 and 3.

Validation on Windows x86_64, CPython 3.11.13, pytest 9.1.1. The compiled path uses the UnityPyBoost extension of the 1.25.4 wheel; UnityPyBoost/ has no changes between 1.25.4 and 417d595.

Check	Result
New tests against unmodified 417d595	2 failed
Each commit's test against its parent commit	fails before, passes after, with and without the extension
Complete suite, with / without the extension	50 passed / 50 passed
ruff check and ruff format --check	Passed

python -m pytest -q tests/test_compressed_mesh.py
python -m pytest -q tests