Emit OSC 8 hyperlinks for links

[brainkim]

This issue was generated by Claude Code.

An a[href] renders underlined in LinkText, but the terminal doesn't know it's a link. Activation does nothing: the engine can't follow a hyperlink, and runActivationBehavior says so. The terminal can, if the painter marks the cells with OSC 8:

ESC ] 8 ; id=<id> ; <url> ESC \  …text…  ESC ] 8 ; ; ESC \

iTerm2, kitty, WezTerm, Ghostty, foot and Alacritty then let the user open the link with a modifier-click or on hover, regardless of mouse reporting, since the terminal handles that click itself. tmux 3.4+ passes OSC 8 through.

What it takes:

• A per-cell link slot in the screen grid, next to the packed attributes, holding an index into a per-frame link table (url + id).
• The diff serializer opens and closes the OSC 8 string around runs whose link changes, the way it does for SGR.
• A stable id per link element across frames, so a link that breaks across lines or is repainted stays one link to the terminal.
• Only absolute http, https, mailto and file URLs; href="#x" and relative URLs mean nothing outside the document. a.href already resolves against the document URL.
• Tests: the bytes for a link, a link wrapped across two lines sharing one id, and no OSC 8 for a placeholder anchor or a fragment link.

Out of scope: following a link in-engine, which stays with the app's click listener.

🤖 Generated with Claude Code

https://claude.ai/code/session_015Bw3yEu4cfkuDkaZ3RSy4D