Add subtle sponsor line: header strip + footer, tracked UTM

css/site.css

@@ -871,3 +871,36 @@ html.js [data-rv].rv-in { opacity: 1; transform: none; }
   font-size: var(--step-sm);
   color: var(--muted);
 }
+
+/* ---------- sponsor line: one quiet line under the masthead, one in the footer ---------- */
+.sponsor { border-bottom: 1px solid var(--rule); }
+.sponsor p {
+  max-width: var(--max);
+  margin: 0 auto;
+  padding: 7px var(--pad) 8px;
+  display: flex;
+  flex-wrap: wrap;
+  align-items: baseline;
+  gap: 8px;
+  font-size: var(--step-sm);
+  color: var(--muted);
+}
+.sponsor-label {
+  font-size: var(--step-xs);
+  letter-spacing: 0.14em;
+  text-transform: uppercase;
+  font-weight: 600;
+  color: var(--oak);
+}
+.sponsor a {
+  color: var(--ink);
+  text-decoration-color: var(--oak);
+  text-underline-offset: 3px;
+}
+.sponsor-note { opacity: 0.72; }
+.sponsor-foot {
+  margin: 10px 0 0;
+  font-size: var(--step-sm);
+  color: var(--muted);
+}
+.sponsor-foot .sponsor-label { margin-right: 6px; }