Use compatible checkbox marker glyphs

[barkure]

Summary

Fixes #747.

• Replace the checked checkbox/menu marker 🗹 (U+1F5F9 BALLOT BOX WITH BOLD CHECK) with ■ (U+25A0 BLACK SQUARE).
• Replace the unchecked checkbox marker ☐ (U+2610 BALLOT BOX) with □ (U+25A1 WHITE SQUARE).
• These replacement glyphs are from the long-standing Unicode Geometric Shapes block, so they should render reliably in common terminal fonts on Windows, Linux, and macOS.
• The previous unchecked marker rendered smaller than the checked marker in some terminals, so the new ■ / □ pair also improves visual consistency between checked and unchecked states.

Happy to use a different marker pair if maintainers prefer another option.

Test plan

• Built successfully with:

  cargo build -p edit --release

• Manually tested the resulting binary on macOS.

[lhecker]

It was actually an intentional decision to avoid ■ and □. Those two glyphs have an "ambiguous" "East-Asian Width". MS Gothic for instance:

As you can see there, these two characters are not 1 cell wide.

What we could use is U+25FC BLACK MEDIUM SQUARE ◼ and U+25FB WHITE MEDIUM SQUARE ◻. Apart from these, it's not looking too good, I think.

[barkure]

Thanks for the context. I switched the markers to U+25FC / U+25FB and verified the updated build locally.

[lhecker]

It is unfortunately necessary to test these new glyphs vs the old ones on a variety of terminals and fonts. If you could test at least a subset of that, it would help me a lot!

[barkure]

I tested the new glyphs against the old ones on a subset of terminals and fonts.

macOS:

• Ghostty
• Terminal.app
• kitty

Fonts tested:

• SF Mono
• Monaco
• JetBrains Mono
• Fira Code

---

Windows:

• Windows Terminal

Fonts tested:

• Consolas
• Cascadia Code
• Courier New
• MS Gothic

I didn't notice any obvious rendering issues in this subset.