cli: export the terminal display-width measure #8863

What is the problem this feature would solve?

#7572 fixed help alignment by measuring text in terminal cells instead of UTF-16 code units, which closed #7557. The measure itself stayed internal (visualLength in CliOutput.ts), so the second half of that issue is still open.

Anything a CLI draws next to the built-in output has to measure text by the same rules to line up with it: a custom CliOutput.Formatter, a boxed banner, a table, a prompt built on Prompt.Custom. Today the only options are copying the grapheme and East Asian Width logic into the application, or adding string-width. Either way the application's measure can disagree with the one the built-in help uses, so the two drift as either side changes.

What is the feature you are proposing to solve the problem?

Export the existing measure, for example as CliOutput.displayWidth:

CliOutput.displayWidth("ファイル") // 8
CliOutput.displayWidth("é")   // 1
CliOutput.displayWidth("1️⃣") // 2

It should strip ANSI styling as it does today, so styled and unstyled text measure the same. No new logic is needed, only the export and a doc comment.

What alternatives have you considered?

• Reimplementing it per application: what we do today. It duplicates Unicode knowledge that already lives in Effect, and the two implementations can disagree.
• string-width: an extra dependency that measures by its own tables, so it can still disagree with the built-in help.
• A home in effect/String instead of effect/cli: also fine, since the measure is useful outside the CLI module.