Fix dataset-ref walk false positive and wire spec-mandated success toasts

- Prune the data payload and top-level datasets keys in all three ref walks
  (extractDatasetRefs, renameDatasetInSpec, resolveDatasetRefs) so a data row
  carrying a field named "data" is no longer misread as a library reference,
  spuriously rewritten, or made to throw DatasetNotFoundError. Adds tests,
  including a guard that lookup-transform refs (from.data) still resolve.
- Wire the deferred success toasts now the Toaster has landed: publish, revert,
  extract-to-dataset, and snippet/dataset delete. Copy follows the council
  title-vs-message rule (title states the action, message adds the consequence).
- Reconcile the spec's blanket toast mandate to "toast only what the user can't
  already see": no toast on visible-result creates (snippet, dataset form);
  Copy Reference stays inline and gains an aria-live announcement (new shared
  .visually-hidden utility) instead of a toast-per-copy.
- Move the toast region to bottom-right so it stops covering the header action
  cluster (Publish/Revert, theme/datasets).
- Update docs/spec 01F/02/05 and docs/architecture/07 + 10 to match.

src/app/components/DatasetsModal.tsx

@@ -191,8 +191,16 @@ function DatasetDetail({
       confirmLabel: 'Delete',
       danger: true,
     });
-    // TODO (M6, spec §05): success toast on delete.
-    if (ok) remove(dataset.id);
+    if (!ok) return;
+    const removedName = dataset.name;
+    remove(dataset.id);
+    // Confirm the deletion (spec §05). The message names which dataset went
+    // (council toast-copy rule, docs/architecture/10 → Toast copy).
+    notify({
+      kind: 'success',
+      title: 'Dataset deleted',
+      message: `"${removedName}" was permanently removed.`,
+    });
   };
 
   return (
@@ -203,6 +211,13 @@ function DatasetDetail({
           <button type="button" className={styles.action} onClick={() => void handleCopy()}>
             {copied ? 'Copied' : 'Copy Reference'}
           </button>
+          {/* The clipboard write is invisible, so the success is confirmed inline
+              ("Copied") rather than by a toast (docs/architecture/10 → Toast copy).
+              This polite live region announces it to assistive tech, which the
+              button's visual label swap alone would not reliably do. */}
+          <span role="status" className="visually-hidden">
+            {copied ? 'Reference copied to clipboard' : ''}
+          </span>
           <button type="button" className={styles.action} onClick={handleEdit}>
             Edit
           </button>