refactor: define VAE tile dimensions in image pixels

[leejet]

Summary

• Use 256×256 image-pixel tiles by default for both VAE encode and decode, independent of the VAE scale factor. Remove the implicit encode size multiplier.
• Rename C API and JSON fields from tile_size_x/y to tile_size_w/h and rel_size_x/y to rel_size_w/h. Keep CLI option names unchanged and document the migration.

Breaking changes: absolute tile sizes now use image pixels instead of latent units, and C/JSON consumers must adopt the enamed fields.

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