test(10.1-04): cover the Phase 10.1 extension point Go code

- acme fixture plugin under modules/cabana/testdata/extension (gadgets
  controller, header and form partials, lookup widget, JS and CSS)
- TestPhase101FormExtensionSchema, TestPhase101PartialSchema and
  TestPhase101Toolbar: every widget, partial and toolbar boot rule
- TestPhase101PartialSanitizer: tag, attribute and URL allowlist, escaping,
  per-request trans, size/node/depth caps and the view-model guard
- TestPhase101Assets: exact-key asset hits, revalidation, SPA fall-through,
  boot path checks and ?v= schema URLs
- TestPhase101Actions (PostgreSQL): scoping, fill filter, strict body,
  action permission, error mapping, CSRF header, toolbar and partial routes
- TestPhase101BoardwalkExports: ContentType and SetSecurityHeaders

modules/cabana/testdata/extension/assets/css/gadgets.css

@@ -0,0 +1,4 @@
+acme-demo-lookup button {
+  font: inherit;
+  color: var(--c-text);
+}

modules/cabana/testdata/extension/assets/js/lookup.js

@@ -0,0 +1,18 @@
+// A plain custom element: one light-DOM button that asks the admin SPA to
+// run the field's action. It makes no request and reads no cookie or
+// storage; the SPA owns HTTP.
+class AcmeDemoLookup extends HTMLElement {
+  connectedCallback() {
+    if (this.firstChild) return
+    const button = document.createElement('button')
+    button.type = 'button'
+    button.textContent = this.getAttribute('label') || ''
+    button.addEventListener('click', () => {
+      this.dispatchEvent(new CustomEvent('summer-action', { bubbles: true, composed: true }))
+    })
+    this.append(button)
+  }
+}
+if (!customElements.get('acme-demo-lookup')) {
+  customElements.define('acme-demo-lookup', AcmeDemoLookup)
+}