dashboard: remove stale KPI box before re-inserting on each poll

Assign a stable id to the KPI element so the previous instance can be
found and removed before the fresh one is prepended to the TOC sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

state-hub/dashboard/src/decisions.md

@@ -151,9 +151,12 @@ const _kpiBox = html`<div class="kpi-infobox">
 withDocHelp(_kpiBox, "/docs/decisions-kpi");
 
 // ── Inject into the TOC sidebar (right column, non-scrolling) ───────────────
+_kpiBox.id = "decisions-kpi-box";
 const _toc = document.querySelector("#observablehq-toc");
-if (_toc) _toc.prepend(_kpiBox);
-else display(html`<div style="float:right;width:215px;margin-left:1rem">${_kpiBox}</div>`);
+if (_toc) {
+  document.getElementById("decisions-kpi-box")?.remove();
+  _toc.prepend(_kpiBox);
+} else display(html`<div style="float:right;width:215px;margin-left:1rem">${_kpiBox}</div>`);
 ```
 
 ```js