Measure and center full multiline equation bounds

[Happypig375]

Summary

• compute recursive ink bounds for multiline displays whose rows extend beyond the root advance
• use aggregate bounds for measurement, alignment, highlighting, and Skia image export only when required
• preserve the historical relayout and highlight path for ordinary single-line displays
• keep constrained and unbounded text centering within the actual multiline ink bounds
• add focused coordinate/bounds tests and reviewed cross-frontend goldens

Stack

This is based on fix/issue-174-centered-text-bounds / #287 because it extends that constrained-text placement work. GitHub's diff for this PR contains only the #239 commit.

Validation

• TestMultilineBounds: 17/17
• TestTextBounds: 9/9
• TestMeasure: 3/3
• complete Rendering suite, all frontends: 1,256/1,256
• scoped format verification and git diff --check
• independent senior review: approved
• independent post-rebase validation: passed

updateBaselines remains disabled; exactly 12 intentional multiline goldens changed. The legacy Magnification baseline is unchanged.

Fixes #239.
Also covers the centered multiline scenario previously reported in #172.