Add focused application-UI layout primitives

[tritao]

Summary

• add weighted GROW sizing with min/max redistribution
• add main-axis distribution, baseline alignment, wrapped flow, row/column gaps, and per-child alignment
• add generic external element measurement with intrinsic metrics and floating logical spacing
• add regression, invariant, measurement, benchmark, and pathological-layout coverage

This intentionally stays an application-UI layout kernel and does not attempt CSS compatibility.

Validation

• NativeKit integration CTest suite: 28/28 passed (joystick test skipped)
• Clay layout benchmarks: 100, 1k, 10k, and 100k node scenarios
• ASan layout tests and 10,000 randomized invariant cases passed

[tritao]

Didn't mean to open this at this time.