Support per-cell tag styling in shared-wall grid rectangles

[zadok-shay]

great project - thanks!
Following my issue.

When rectangles share walls in a grid (connected via + intersections), svgbob treats the entire grid as a single group of lines rather than individual closed shapes.
This means {tag} markers inside grid cells are rendered as plain text instead of being recognized as CSS class tags.

+------+------+
| {a}  | {b}  |
| Foo  | Bar  |
+------+------+

# Legend:
a = {
    fill: lightblue;
}
b = {
    fill: lightyellow;
}

above will not be rendered with colors

I kindly request adding it - if possible

Thanks!

---

the WA I use

+------+ +------+
| {a}  | | {b}  |
| Foo  | | Bar  |
+------+ +------+

# Legend:
a = {
    fill: lightblue;
}
b = {
    fill: lightyellow;
}