[css-gaps-1] Introduction of `auto` for `rule-break` properties.

[jav099]

We would like to introduce an auto keyword for the rule-break properties.

The goal is for auto to become the initial value for all rule-break longhands, with its resolved value depending on the layout type and axis. This makes the behavior future-extensible across layout models, and allows us control the default behavior to preserve reading behavior across different layout types.

Proposed resolution behavior

Grid containers

For grids, the default behavior would align naturally with how spanning items interrupt gap decorations:

• rule-break: auto
  • column-rule-break: auto → spanning-item
  • row-rule-break: auto → spanning-item

Flex containers

Flex layouts do not have spanning behavior, so auto resolves to no interruptions:

• rule-break: auto
  • column-rule-break: auto → none
  • row-rule-break: auto → none

Multicol containers

For multicol layouts, auto would currently resolve the same as Flex (pending resolutions in related issues):

• rule-break: auto
  • column-rule-break: auto → none
    (subject to change depending on future multicol decisions)
  • row-rule-break: auto → none

Even though today many of these resolved values would look similar across layout types, defining auto now makes the model extensible if new break behaviors or layout models are added in the future.