fix(assets): decode normalized glTF skin weights

[Changdw]

修复 glTF importer 未正确处理 accessor.normalized 导致的 skin weights 导入问题。

问题场景：

• UE5 glTF Exporter 会导出 WEIGHTS_0/WEIGHTS_1 为 UNSIGNED_BYTE + normalized: true。
• 旧逻辑直接按原始整数读取权重，随后在 reduceJointInfluences() 中对 Uint8Array 做归一化，浮点结果被截断，导致权重损坏，蒙皮网格出现尖刺。
• 这个问题在 cocos/engine-extensions 中同样存在，因为 cocos-cli 的这段 glTF importer 代码来源于 Creator 的 engine-extensions。

修复内容：

• 对 normalized: true 的 WEIGHTS_* accessor 在进入 joint influence reduction 前解码为 Float32Array。
• 保留旧资产兼容性：normalized: false 或未声明时不覆盖 PPGeometry 原有的 normalized 推断逻辑。
• 复用原有整数归一化 helper，避免动画 sampler 和 mesh weights 使用两套转换逻辑。

[Changdw]

Fixes glTF skin weight corruption caused by ignoring accessor.normalized.

Problem:

• UE5's glTF Exporter can write WEIGHTS_0/WEIGHTS_1 as UNSIGNED_BYTE with normalized: true.
• The previous importer read those weights as raw integers. When reduceJointInfluences() later normalized them inside a Uint8Array, fractional values were truncated and the skin weights were destroyed, causing spiked skinned meshes.
• The same issue also exists in cocos/engine-extensions, since this glTF importer code in cocos-cli comes from Creator's engine-extensions.

Changes:

• Decode normalized: true WEIGHTS_* accessors to Float32Array before joint influence reduction.
• Preserve legacy asset compatibility: when normalized is false or omitted, the importer does not override PPGeometry's existing normalized inference.
• Reuse the existing integer-to-float normalization helper instead of keeping a separate conversion path.

[Changdw]

#630