M6: Inspect tab: properties with styles and variables, redlines on hover (#41)

The Inspect tab lists the selected layer's properties for handoff:
layout (size, position from the parent, rotation, opacity, radius), auto
layout (direction, gap, padding, alignment), typography, fills, strokes
and effects, each with the style or variable it comes from. Clicking a
value copies it. With the Inspect tab open, hovering measures distances
from the selection without holding Alt.

New command inspect.properties returns the same groups for agents.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -81,6 +81,7 @@ fn registry() -> &'static Registry {
         list.extend(crate::vectoredit::specs());
         list.extend(crate::shapebuilder::specs());
         list.extend(crate::codegen::specs());
+        list.extend(crate::devinspect::specs());
         let mut index = HashMap::new();
         for (i, s) in list.iter().enumerate() {
             if index.insert(s.id, i).is_some() {