Preserve inferred grid visual placement

[chubes4]

Summary

Order inferred flow children by stable source geometry and preserve source-relative placement for inferred freeform grids.

What changed

• Added stable inferred grid/row child ordering while preserving declared auto-layout and layered children.
• Added bounded residual grid-item margins for inferred freeform grids whose asymmetric source geometry cannot be represented by equal CSS tracks.
• Added contracts for visual order, stable ties, explicit auto-layout, absolute/decorative exclusions, source insets, and unequal item placement.

How to test

1. Run cd figma-transformer && composer test; expect Contract suite passes.
2. Run git diff --check; expect No whitespace errors are reported.

Compatibility

Backwards compatibility: declared Figma auto-layout, non-inferred containers, and absolute/decorative/layered child behavior are unchanged.

Evidence

• CI expected: Repository pull request checks
• Reviewer-resolvable source evidence: Improve Figma transformer parity from three-file matrix #608
• contracts: Passed
• diff-check: Passed
• fixture-matrix: Passed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode
• Model: openai/gpt-5.6-sol
• Used for: Recovered the Homeboy candidate, diagnosed its real-fixture gap, implemented the bounded geometry correction and contracts, and verified the fixture matrix with Chris.

Source relationships

• Closes Improve Figma transformer parity from three-file matrix #608