💡 Signed margin: offset a box without consuming content width

[natemoo-re]

Is your feature request related to a problem?

There's no way to offset a box's position within its parent's flow. The only per-side knob is layout.padding, which shrinks the content box instead of moving the box: "Hello W" (7 cells) fits on one line in a fixed(8) box, but padding: { left: 2 } drops content to 6 cells and wraps to two lines. Padding also can't be negative—each side is an unsigned byte, so padding: { left: -3 } wraps to garbage (the box returns height: 511). There's no primitive for placing a box before its parent's content origin and clipping the overflow.

Describe the solution you'd like

A signed layout.margin that moves a box without shrinking its content, mirroring the CSS margin property: it sits outside the content box and may be negative. margin: { left: 2 } keeps the box's width and stays on one line; a negative left margin places the origin before the parent's content origin, so with the parent clipping, the leading cells are suppressed and the rest renders flush to the edge.

open("box", {
  layout: { width: fixed(12), margin: { left: -3 } },
});

Describe alternatives you've considered

padding as a stand-in fails both ways: it shrinks content and corrupts geometry on negative input (unsigned packing). Pre-clipping strings in JS can't reflow or re-clip on resize and doesn't generalize to nested boxes.

Additional context

Failing test case on nm/repro/signed-margin (test · diff). margin needs its own signed word (the ops.ts padding packing is 4×uint8 and can't be reused) and a post-layout offset step in the layout decode (src/clayterm.c:499); Clay_LayoutConfig has no margin, so it's a new primitive on top of Clay that leans on the existing clip path.