feat: comptime CSS-in-Zig — type-safe styles with dead code elimination

[justrach]

Problem

Page CSS is currently raw multiline strings with no validation:

const page_css = 
    \\.card { background: var(--bg2); }
    \\.card:hover { opacity: 0.88; }
;

Typos in class names, unused styles, and invalid properties are never caught.

Proposed Solution

A comptime CSS builder that validates at compile time:

const styles = mer.css(.{
    .card = .{ .background = "var(--bg2)", .border_radius = "8px" },
    .card_hover = .{ .opacity = 0.88 },
    .title = .{ .font_size = "clamp(28px, 4vw, 42px)", .font_family = "serif" },
});

// In render:
h.div(.{ .class = styles.card }, .{ ... })
// ^^^ compile error if you reference styles.nonexistent

Benefits:

• Dead style elimination — only emit CSS for classes actually used
• Property validation at comptime (catch backgroud typos)
• Scoped class names to prevent collisions between pages
• CSS output is a single minified string

Complexity

Medium — Zig's comptime is powerful enough to build this as a library, no compiler changes needed.