feat: drive layer split from graph-cut segments

[leejet]

Summary

• Rework layer split to defer assignment until the first graph and reuse mark_graph_cut segments for parameter/backend placement.
• Add model-manager support for reassigning compute backends before params are loaded, including params-backend-follow-runtime handling.
• Propagate graph-cut layer split and per-backend --max-vram limits through text encoder conditioners.

Related Issue / Discussion

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Additional Information

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Checklist

• I have read and confirmed this PR follows the contribution guidelines.