Route bounded triangle meshes through Scene exports

[DecisionNerd]

Summary

• route literal, unjoined, constant-color triangle meshes through the public Scene export path
• enforce the browser painter group budget at the per-trace and mixed-figure boundaries
• preserve compatibility fallbacks for joined, styled, polar, secondary-axis, implicit-domain, and non-finite cases
• update authoritative Scene ownership and compatibility specifications

Validation

• Rust Scene tests (51 passed)
• release xyg-core build
• affected Python tests (198 passed)
• full Node tests (198 passed)
• browser render smoke
• Scene ownership verifier (190 files)
• Ruff check and format check
• full pre-commit suite

Advances #58 and #116 without closing either.

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