printedparts docs: record the open-write kiosk decision; defer the dashboard widget

The kiosk take endpoint is the product's first unauthenticated
mutation; spell out the acceptance criteria (decrement-only, badge
attributed, bounded, physically rate-limited) so future open-write
endpoints meet the same bar. The dashboard-widget milestone is marked
optional: get_dashboard_widgets predates the ADR-010 data-only
renderers and needs a core component to render.

docs/PLUGIN-LAB-PRINTEDPARTS.md

@@ -166,7 +166,11 @@ Build
 2. Stock report includes the reconcile check: flag rows where cached
    `quantityonhand` != SUM(ledger). Should always be empty; if not, you have
    a non-atomic write path - find it.
-3. `get_dashboard_widgets()` -> low-stock count.
+3. OPTIONAL/deferred: `get_dashboard_widgets()` -> low-stock count. Caveat:
+   this hook predates the ADR-010 data-only renderers - the widget names a
+   frontend component that must already exist in core, so a plugin widget
+   only renders if you also add that component. Reports are the primary
+   monitoring surface; skip the widget unless you want the extra credit.
 4. Nice-to-have if time: burn rate (avg weekly takes over trailing 4 weeks +
    weeks-to-empty). Plain SQL over the ledger.