fix(ui): cache complex Unicode cluster widths

[benvinegar]

Summary

• segment composition-sensitive text once instead of measuring the full line with string-width
• cache exact multi-scalar cluster widths with a 256-entry / 64-code-unit FIFO bound
• add cache-bound and Unicode-parity coverage plus a patch changeset

Validation

• bun test src/ui/lib/ui-lib.test.ts
• bun run typecheck
• bun run lint
• bun run bench:terminal-width — checksum remains 196000; complex-cluster hot path measured ~28.5× faster locally

bun test still has unrelated fresh-main failures in review-conformance/diff pairing plus Playwright test discovery.

This PR description was generated by Pi using gpt-5.6-terra

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
hunk-web	Ignored		Aug 18, 2026 2:30am

[greptile-apps]

Greptile Summary

This PR optimizes complex Unicode width measurement by segmenting composition-sensitive text once and caching exact multi-scalar cluster widths.

• Adds a 256-entry FIFO cache with a 64-code-unit key limit.
• Preserves string-width parity while avoiding repeated full-line segmentation.
• Adds cache-boundary and Unicode-parity tests plus benchmark documentation and a patch changeset.

Confidence Score: 5/5

The PR appears safe to merge with no concrete blocking or non-blocking defects identified.

The bounded cache preserves exact cluster widths, enforces its intended entry and key-size limits, and the new segmented traversal remains equivalent to the existing string-width behavior for reachable sanitized text.

Important Files Changed

Filename	Overview
src/ui/lib/text.ts	Adds the bounded cluster-width cache and routes composition-sensitive width calculation through a single grapheme traversal without an identified correctness regression.
src/ui/lib/ui-lib.test.ts	Adds focused coverage for FIFO eviction, oversized-key rejection, cache churn, and Unicode width parity.
benchmarks/terminal-width.ts	Updates benchmark terminology to describe the cached complex-cluster path.
benchmarks/README.md	Updates terminal-width benchmark documentation to reflect cached cluster measurement.
.changeset/swift-clusters-measure.md	Records the Unicode width-measurement performance improvement as a patch release.

_{Reviews (1): Last reviewed commit: "fix(ui): cache complex cluster widths" | Re-trigger Greptile}