Fix page merging into writer pages and add layer control

Fix page merging into writer pages and add layer control: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). produces a blank output page: the merged content and its resources (fonts, images, graphics states, …) are lost once the document is written out and read back…

The task

produces a **blank** output page: the merged content and its resources (fonts, images, graphics states, …) are lost once the document is written out and read back. The merged content is only visible while everything stays in memory.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.