introduce tags, structs, and reticles

this was meant to be split into smaller changes, but I realised I edited my existing revision too late.

crates/haku/src/render.rs

@@ -127,7 +127,7 @@ impl<'a> Renderer<'a> {
             &paint,
             &SStroke {
                 width: stroke.thickness,
-                line_cap: LineCap::Square,
+                line_cap: LineCap::Round,
                 ..Default::default()
             },
             transform,