Chart Builder: open a snippet in the builder to edit it in place

src/app/components/ChartBuilderModal.module.css

@@ -58,6 +58,19 @@
   gap: var(--space-3);
 }
 
+/* Edit-in-place banner (spec §06 → Open in builder): names the snippet the builder
+   is editing, so the primary "Save changes" action reads unambiguously. */
+.editingBanner {
+  margin: 0;
+  font-size: 12px;
+  color: var(--text-secondary);
+}
+
+.editingBanner strong {
+  color: var(--text-primary);
+  font-weight: 600;
+}
+
 /* ── Intent front door (spec §06 → Intent) ───────────────────────────────
    A persistent "what do you want to show?" strip under the dataset picker. A
    quiet accent-soft wash marks it as the guided on-ramp without competing with