input: gutter cursor style + line hitbox accessors

[glani]

Problem

Two small gaps for editors that show a line-number gutter:

1. The mouse cursor stays a text I-beam over the gutter, so the
   line-number column reads as part of the text-insertion area even
   though it isn't.
2. A consumer has no way to find where the gutter is on screen, or where
   a given buffer line is, without re-deriving layout — which it needs
   for things like click-to-select-line, breakpoint toggles, or placing
   a popover next to a specific line.

Solution

Gutter cursor. When the mode shows line numbers, paint sets
CursorStyle::Arrow over the line-number hitbox so the gutter no longer
reads as the text-insertion area. Gated on InputMode::line_number(), so
single-line and auto-grow inputs are unaffected.

Two read-only accessors on InputState:

pub fn line_number_hitbox(&self) -> Option<&Hitbox>;
pub fn visible_line_bounds(&self, row: u32) -> Option<Bounds<Pixels>>;

• line_number_hitbox() exposes the gutter hitbox captured during the
  last paint.
• visible_line_bounds(row) returns the on-screen bounds of a buffer row
  from the last layout, soft-wrap aware.

Both are primitives for consumer-side click routing. The only behavior
change is the mouse cursor over the gutter (which previously had no
special cursor); everything else is strictly additive.