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

src/app/components/ChartBuilderModal.tsx

@@ -1466,6 +1466,12 @@ export function ChartBuilderModal() {
   const setStack = useChartBuilderStore((s) => s.setStack);
   const applyWarningFix = useChartBuilderStore((s) => s.applyWarningFix);
   const runCreate = useChartBuilderStore((s) => s.createSnippet);
+  const runSave = useChartBuilderStore((s) => s.saveEdits);
+  // Edit-in-place (spec §06 → Open in builder): when a snippet was opened in the
+  // builder, the primary action *saves back* to it instead of creating a new snippet.
+  const editingSnippetId = useChartBuilderStore((s) => s.editingSnippetId);
+  const editingSnippetName = useChartBuilderStore((s) => s.editingSnippetName);
+  const editing = editingSnippetId !== null;
 
   // Validity + guidance + which chart-level controls apply are derived from the
   // stable `config` reference via useMemo, NOT a store selector that would build a
@@ -1526,6 +1532,11 @@ export function ChartBuilderModal() {
         <div className="visually-hidden" role="status" aria-live="polite">
           {fixAnnouncement}
         </div>
+        {editing && (
+          <p className={styles.editingBanner}>
+            Editing <strong>{editingSnippetName}</strong>
+          </p>
+        )}
         <DatasetPicker datasetId={datasetId} />
 
         <IntentStrip />
@@ -1610,7 +1621,7 @@ export function ChartBuilderModal() {
 
         {!valid && (
           <p id="cb-create-hint" className={styles.createHint}>
-            Map at least one channel to a column to create a snippet.
+            Map at least one channel to a column to {editing ? 'save changes' : 'create a snippet'}.
           </p>
         )}
         <div className={styles.actions}>
@@ -1622,12 +1633,12 @@ export function ChartBuilderModal() {
             size="lg"
             disabled={!valid}
             aria-describedby={!valid ? 'cb-create-hint' : undefined}
-            // The create is the user's confirmation — close with no discard prompt.
+            // The commit is the user's confirmation — close with no discard prompt.
             onClick={() => {
-              if (runCreate()) void closeModal(true);
+              if (editing ? runSave() : runCreate()) void closeModal(true);
             }}
           >
-            Create Snippet
+            {editing ? 'Save changes' : 'Create Snippet'}
           </Button>
         </div>
       </div>