fix: handle NaN layout widths

[hiSandog]

Summary

• Normalize NaN maxWidth inputs to zero before running public layout APIs
• Keep layout(), layoutWithLines(), layoutNextLine(), layoutNextLineRange(), walkLineRanges(), and measureLineStats aligned for transient invalid width measurements
• Add a regression test covering the NaN fallback behavior

Tests

• bun test src/layout.test.ts
• bun run check
• git diff --check