Remove offsets from Tokenizer interface

[sagearc]

Render and RenderChat in the Tokenizer interface return ([]uint32, []types.Offset, error), but since switching to the renderer service (#461) these methods always return nil offsets. The offset return value is now dead weight — all callers already discard it with _.

Should drop []types.Offset from both signatures in the interface and all implementations (UdsTokenizer, CachedTokenizer, CompositeTokenizer, mocks).