Implement 8-bit ANSI

[clipperhouse]

Based on this discussion with @aymanbagabas, implement “plain” 8-bit ANSI initiators, instead of my notion of “UTF-8 encoded” initiators. This represents a mode for interpreting a raw terminal stream, as opposed to assuming that everything coming in is valid UTF-8.

This is a bit of a departure / scope creep for this graphemes package, but the opportunity is to allow charmbracelet/x/ansi to have faster / simpler width calculations. This PR takes care to test for identical boundaries between their parser and mine.

Here are some preliminary local benchmarks, just iterating, not doing anything else:

goos: darwin
goarch: arm64
pkg: github.com/clipperhouse/uax29/graphemes/comparative
cpu: Apple M2
BenchmarkAnsiIteration/clipperhouse/uax29-8      	26598 ns/op	   506.80 MB/s	  0 B/op	0 allocs/op
BenchmarkAnsiIteration/charmbracelet/x/ansi-8       42117 ns/op	   320.06 MB/s	  0 B/op	0 allocs/op

[Copilot]

Pull request overview

This PR implements 8-bit ANSI escape sequence support by treating raw bytes 0x80-0x9F as C1 control codes, replacing the previous UTF-8-encoded C1 approach. The change is motivated by enabling faster ANSI width calculations in charmbracelet/x/ansi by working with raw terminal streams rather than assuming all input is valid UTF-8.

Changes:

• Switched from UTF-8-safe C1 detection (0xC2 0x80-0x9F) to raw byte detection (0x80-0x9F)
• Updated ANSI parsing to handle single-byte C1 controls directly instead of two-byte UTF-8 sequences
• Added comprehensive boundary agreement tests comparing against charmbracelet/x/ansi's parser

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
graphemes/iterator.go	Changed ANSI detection from UTF-8-encoded C1 (checking for 0xC2 lead byte) to raw bytes 0x80-0x9F, added ST constant
graphemes/ansi.go	Refactored to handle 8-bit C1 controls as single bytes instead of UTF-8 two-byte sequences; updated terminator detection
graphemes/ansi_test.go	Updated all test inputs from UTF-8-encoded C1 sequences (\xC2\x9B) to raw bytes (\x9B); removed UTF-8 validation test
graphemes/comparative/comparative_test.go	Added comprehensive boundary agreement tests against charmbracelet/x/ansi parser with 60+ test cases
graphemes/comparative/go.mod	Updated Go version to 1.24.2 and added charmbracelet/x/ansi dependency
graphemes/comparative/go.sum	Added checksums for new dependencies

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 1 comment.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.