perf(core): cache ribbon_groups() with OnceLock, avoid per-frame tree allocations

Change CadModule::ribbon_groups() from -> Vec<RibbonGroup> to -> &[RibbonGroup],
backed by a per-module OnceLock cache. The ribbon group tree is static data
(&'static str, Copy enums) that was being fully reconstructed — Vecs, String
clones, enum discriminants — on every call, including all three per-frame
call sites (view(), dropdown_overlay(), style_combo_overlay()).

- 7 built-in modules + 2 plugin templates cache via function-local OnceLock.
  Safe because each is a unit struct with exactly one instance per process
  (plugins run in isolated child processes; see PluginProcess::spawn()).
- SharedCadModule avoids the static pattern entirely, storing groups in an
  instance field (owned.rs).
- IPC runner converts &RibbonGroup -> OwnedRibbonGroup directly via new
  From<&T> impls, avoiding an intermediate clone.
- render_small/render_large and the two make_tool_row closures now borrow
  (&RibbonItem, &[ToolDef]) instead of taking ownership, so the view loop
  doesn't need to clone items to satisfy the old by-value signatures.
- Various match-ergonomics deref fixes (*id, *default, *icon, etc.) from
  the &RibbonItem pattern change.

src/modules/layout/mod.rs

@@ -36,31 +36,36 @@ impl CadModule for LayoutModule {
         "Layout"
     }
 
-    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
-        vec![
-            RibbonGroup {
-                title: "Viewport",
-                tools: vec![mview::tool().into()],
-            },
-            RibbonGroup {
-                title: "Plot",
-                tools: vec![
-                    ToolDef {
-                        id: "PAGESETUP",
-                        label: "Page Setup",
-                        icon: IconKind::Svg(include_bytes!("../../../assets/icons/pagesetup.svg")),
-                        event: ModuleEvent::Command("PAGESETUP".to_string()),
-                    }
-                    .into(),
-                    ToolDef {
-                        id: "PLOT",
-                        label: "Export PDF",
-                        icon: IconKind::Svg(include_bytes!("../../../assets/icons/plot.svg")),
-                        event: ModuleEvent::Command("PLOT".to_string()),
-                    }
-                    .into(),
-                ],
-            },
-        ]
+    fn ribbon_groups(&self) -> &[RibbonGroup] {
+        static GROUPS: std::sync::OnceLock<Vec<RibbonGroup>> = std::sync::OnceLock::new();
+        GROUPS.get_or_init(|| {
+            vec![
+                RibbonGroup {
+                    title: "Viewport",
+                    tools: vec![mview::tool().into()],
+                },
+                RibbonGroup {
+                    title: "Plot",
+                    tools: vec![
+                        ToolDef {
+                            id: "PAGESETUP",
+                            label: "Page Setup",
+                            icon: IconKind::Svg(include_bytes!(
+                                "../../../assets/icons/pagesetup.svg"
+                            )),
+                            event: ModuleEvent::Command("PAGESETUP".to_string()),
+                        }
+                        .into(),
+                        ToolDef {
+                            id: "PLOT",
+                            label: "Export PDF",
+                            icon: IconKind::Svg(include_bytes!("../../../assets/icons/plot.svg")),
+                            event: ModuleEvent::Command("PLOT".to_string()),
+                        }
+                        .into(),
+                    ],
+                },
+            ]
+        })
     }
 }