✨ add text measurement helpers

[rauhryan]

Summary

• add pure TypeScript text measurement helpers exported from mod.ts
• document helper API and large-input behavior in specs/renderer-spec.md
• normalize oversized text packing failures to descriptive RangeError messages
• add coverage for wrap modes, Unicode widths, no-WASM/no-UTF8-staging measurement, and large UTF-8 projection rendering

Test

• deno test

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/clayterm@30

commit: 6e3fcd6

[cowboyd]

I'm a bit worried that these measurement helpers present a completely different code path than the actual algorithm that clay uses to layout text. This could be a totally separate library.