Improve performance of maxRuneWidth

[clipperhouse]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

A little optimization on maxRuneWidth, looks like ~2x.

goos: darwin
goarch: arm64
pkg: github.com/charmbracelet/lipgloss
cpu: Apple M2

BenchmarkMaxRuneWidth/Blank/Before-8  	       6.832 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaxRuneWidth/Blank/After-8   	       2.110 ns/op	       0 B/op	       0 allocs/op

BenchmarkMaxRuneWidth/Markdown/Before-8        6.755 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaxRuneWidth/Markdown/After-8         2.060 ns/op	       0 B/op	       0 allocs/op

BenchmarkMaxRuneWidth/Normal/Before-8         32.11 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaxRuneWidth/Normal/After-8          14.55 ns/op	       0 B/op	       0 allocs/op

BenchmarkMaxRuneWidth/Rounded/Before-8        30.79 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaxRuneWidth/Rounded/After-8         14.57 ns/op	       0 B/op	       0 allocs/op

BenchmarkMaxRuneWidth/Block/Before-8          33.84 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaxRuneWidth/Block/After-8           14.65 ns/op	       0 B/op	       0 allocs/op

BenchmarkMaxRuneWidth/Emoji/Before-8          29.45 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaxRuneWidth/Emoji/After-8           15.38 ns/op	       0 B/op	       0 allocs/op

[Copilot]

Pull request overview

This PR optimizes the maxRuneWidth function in lipgloss by replacing the uniseg package with clipperhouse/displaywidth, achieving approximately 2x performance improvement. The optimization includes fast-path handling for empty and single-byte strings, along with a more efficient grapheme iteration approach for multi-character strings.

• Replaced uniseg.FirstGraphemeClusterInString with displaywidth.StringGraphemes for better performance
• Added fast-path optimizations for empty and single-byte strings
• Simplified getBorderEdgeWidth implementation using built-in max() function

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
borders.go	Refactored maxRuneWidth with optimized implementation using displaywidth package and added fast paths; simplified getBorderEdgeWidth to use max()
borders_test.go	Added comprehensive benchmark comparing old vs new maxRuneWidth implementations with various character types; retained old implementation for comparison
go.mod	Added github.com/clipperhouse/displaywidth v0.6.0 dependency
go.sum	Added checksums for new displaywidth dependency and its transitive dependencies (stringish, uax29)

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