fix: split batches by castShadow and shadowCascadeMask

[mvaligursky]

When mesh instances were merged into one batch, the resulting single MeshInstance took castShadow (and related state) only from the first source instance. Instances that matched on material, shader defines, and other batch keys but differed in shadow casting could all inherit the wrong flag, which showed up as incorrect shadow maps (see #8646).

Changes

• Split batch groups in BatchManager.prepare when castShadow or shadowCascadeMask differs between candidates.
• Document the new split rule in prepare JSDoc.
• Add a unit test that two batched boxes with different castShadow produce two world-layer mesh instances with the expected flags.

API changes

• None.

Performance

• Slightly more batches (and draw calls) when the same batch group mixes shadow-casting and non–shadow-casting geometry; behavior matches disabling batching for those cases.

Fixed #8646