fix(core): wrap oversized text in fixed-width boxes

[sean0407]

Summary

textLayout
  wrap at whitespace boundaries
+ split oversized tokens at grapheme boundaries when a fixed width is set

Fixes #25.

Evidence

• Before: Against upstream main, both new regression cases fail: an unbroken URL stays on one line, and the resulting grapheme text exceeds the fixed layout width.
• After: npm test passes (150 Vitest tests and 3 sync relay tests); npm run typecheck passes; npm run build completes.

Merge Danger

Door: Two-way

The change only affects measured wrapping when a fixed width is set; autosized text keeps its existing path.

Blast Radius: Contained

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