Kitty graphics protocol options serialization drops negative z-index

### Description

The Kitty graphics protocol supports negative z-indexes, with anything between -1 and INT32_MIN/2 (inclusive) meaning "under the text", and anything below INT32_MIN/2 meaning "also under cells with non-default background colors". That's in what is currently the fourth paragraph of the ["Controlling displayed image layout"](https://sw.kovidgoyal.net/kitty/graphics-protocol/#controlling-displayed-image-layout) section of the graphics protocol spec.

The serialization code for kitty.Options however ignores any non-positive z-index. This means that code that should produce an interface that looks like

instead produces

### Version

v0.11.7 and master

### Environment

ghostty and kitty and iterm2

Start by locating the kitty.Options serialization code and compare its z-index handling with the negative ranges described in the linked graphics protocol specification. Add regression coverage for negative z-index values and verify that serialization preserves all supported values without changing positive z-index behavior.

Written by the indexing model from the issue text.