editor: add clickable breakpoint gutter

[gintsgints]

Summary

Adds an optional clickable breakpoint gutter to the multi-line editor (InputState), rendered as dots to the left of the line numbers — like the debug gutter in a code editor.

• Set breakpoints render as a filled dot (theme danger).
• Hovering the gutter reveals faint dots on every line so any line can be clicked to add a breakpoint.
• Clicking a dot toggles the breakpoint and emits InputEvent::BreakpointToggled(line) (0-based buffer row).

API

InputState:

• breakpoints_enabled(bool) — builder to enable the gutter (requires line numbers).
• set_breakpoints_enabled(bool, cx) / is_breakpoints_enabled() — runtime toggle + getter.
• breakpoints() / set_breakpoints(set, cx) — read/replace the breakpoint lines.
• toggle_breakpoint(line, cx) — toggle a single line.

InputEvent::BreakpointToggled(usize) — emitted on toggle.

Notes

Gutter width reserves BREAKPOINT_DOT_WIDTH only when enabled, so existing editors are unaffected. Layout/paint mirrors the existing fold-icon path (layout_fold_icons).

AI-assisted; reviewed and refactored to match project style.

[huacnlee]

I don't think this is a good API design.

[gintsgints]

I don't think this is a good API design.

You think that gutter idea is bad at all or you think realisation need improvement?