refactor(menubar): concentric trigger radius via [data-shape-nest] (A2)

The in-bar trigger hand-rolled its concentric radius
(`calc(--_menubar-radius − --_menubar-padding)`). Consume the shared shape-engine
rule instead: the bar exposes its radius + padding as `--shape-outer-radius` +
`--shape-nest-gap`, the trigger stamps `data-shape-nest`, and the global
`[data-shape-nest]` rule computes the concentric radius once.

No-op verified in-browser: trigger = 2px at md (same as before); identical across
sizes via the var cascade. First of the A2 concentric migration (§13 — stop
hand-rolling `calc(--radius − pad)` per component).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/uix/eidos/components/menubar/menubar-trigger.svelte

@@ -12,6 +12,9 @@
 	let { children, ...rest }: MenubarTriggerProps = $props();
 </script>
 
-<Menubar.Trigger {...rest}>
+<!-- `data-shape-nest`: the in-bar trigger's radius is concentric with the bar —
+	the shared shape-engine rule computes it from the bar's --shape-outer-radius
+	(its radius) minus --shape-nest-gap (its padding). No hand-rolled calc. -->
+<Menubar.Trigger data-shape-nest {...rest}>
 	{@render children?.()}
 </Menubar.Trigger>

src/uix/eidos/components/menubar/menubar.css

@@ -29,6 +29,11 @@
 	--_menubar-radius: var(--radius-md);
 	--_menubar-trigger-px: var(--space-3);
 	--_menubar-font-size: var(--font-size-sm);
+	/* Concentric nesting (Arq. 6 / shape engine): the in-bar trigger consumes the
+	   shared `[data-shape-nest]` rule instead of hand-rolling its own calc — the bar
+	   exposes its radius + padding as the nest's outer-radius + gap. */
+	--shape-outer-radius: var(--_menubar-radius);
+	--shape-nest-gap: var(--_menubar-padding);
 
 	display: inline-flex;
 	align-items: center;
@@ -66,7 +71,8 @@
 	min-block-size: calc(var(--_menubar-height) - var(--_menubar-padding) * 2 - 2px);
 	padding-inline: var(--_menubar-trigger-px);
 	border: none;
-	border-radius: calc(var(--_menubar-radius) - var(--_menubar-padding));
+	/* Radius from the shared `[data-shape-nest]` rule — concentric with the bar
+	   (--shape-outer-radius − --shape-nest-gap, set on [data-menubar] above). */
 	background: transparent;
 	color: var(--color-content-primary);
 	font-family: var(--font-ui);