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);
+}