Replace Table<Table<Graphic>> with Table<Artboard> where Artboard is a type boundary newtype

[Keavon]

This follows the same approach taken by the Graphic struct which is an enum over multiple wrapped types acting as a boundary for the boundless type explosion from infinite nestings of Table<Table<...<all graphical types>>>. Now, Artboard is a newtype script that wraps Table<Graphic> just as Graphic has arms that wrap Table<all graphical types>. This represents an explicit loss of data purity that takes the form of a new named base type, not simply an extra nesting level of the base type.

Followup to #4077. Partly closes #3779.

[cubic-dev-ai]

No issues found across 16 files

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[gemini-code-assist]

Code Review

This pull request introduces a nominal Artboard struct as a wrapper around Table, replacing the previous nested Table<Table> type throughout the codebase. This architectural change improves type safety and clarity by establishing a clear boundary for artboard content. The modifications span the node registry, rendering logic, serialization, and various node implementations to support the new Table type. I have no feedback to provide as there were no review comments to assess.