perf: reuse visible word measurements

[jan-kubica]

Summary

• reuse the visible-word width when deriving the width of ordinary trailing whitespace
• preserve exact full-string measurement for kerning and literal tabs
• account for letter spacing and horizontal scaling at the split boundary
• add focused measurement-count and typography regressions

PR #482 by @arthrod identified redundant text measurement as an optimization direction. This PR takes a smaller dependency-free slice while retaining the existing line-breaking algorithm.

Performance

• focused eight-word prose case: 17 to 10 Canvas probes (41% fewer)
• 20-document DOCX sample: 64,854 to 43,769 probes (32.5% fewer), with identical pagination (838 pages)

The sampled end-to-end wall time remained flat/noisy, so this PR claims fewer Canvas measurements rather than a document-load speedup.

Testing

• bun test src/layout-engine/measure/measureParagraph.test.ts (117 pass)
• bun --filter @stll/folio-core test (4,386 pass, 2 expected skips)
• bun --filter @stll/folio-core typecheck
• targeted oxlint and oxfmt checks

CC on behalf of jan-kubica

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📥 Commits

Reviewing files that changed from the base of the PR and between dafe214 and 8098903.

📒 Files selected for processing (3)

• .changeset/quick-words-measure.md
• packages/core/src/layout-engine/measure/measureParagraph.test.ts
• packages/core/src/layout-engine/measure/measureParagraph.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/reuse-word-measurements

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