Top-row chrome: composer-matching 16px corners, equal 8px gutters

- Corner rounding back (owner, 2026-09-20): the four controls return to
  rounded-xl — 16px in this theme, identical to the composer shell's
  rounded-2xl (verified computed-style equal). Inner elements (search
  input, import button, cost hover) stay concentric: 16 - 4px padding
  = rounded-lg (12px), the composer's own concentric math.
- Equal spacing: the cluster pins 8px from BOTH edges (top-2 + right-2),
  still flush with the sidebar toggle's header band (top 8px).

components/graph-coverage.tsx

@@ -102,7 +102,7 @@ export function GraphCoverageControl({ refreshKey = 0 }: { refreshKey?: number }
 				title="Extraction coverage — what the knowledge graph has covered"
 				aria-expanded={open}
 				onClick={() => setOpen((o) => !o)}
-				className={`flex size-9 items-center justify-center rounded-sm border bg-card/85 shadow-lg backdrop-blur transition-colors hover:bg-accent ${
+				className={`flex size-9 items-center justify-center rounded-xl border bg-card/85 shadow-lg backdrop-blur transition-colors hover:bg-accent ${
 					open ? "text-foreground" : "text-muted-foreground hover:text-foreground"
 				}`}
 			>