feat(16.2-01): compile, decode and load plugin field types in cabana

- compileFieldTypes builds a per-boot field type registry (alias, core and
  cross-plugin collisions, element prefix, assets, option keys) passed down
  the compile functions; settings and relation forms refuse plugin aliases
- fields.yaml accepts a plugin alias with the general keys plus the type's
  options (typeOptions, element, translatable in the schema); CheckOptions
  runs at boot; CompileFormWithTypes for plugin tests
- save decodes plugin values inside the transaction (FormFieldError is a
  422, other errors opaque 500) and answers meta.notices (FieldNotice) only
  when a type reported something; show and save serve values through Load
- GET /field-types lists the types with versioned asset URLs; OpenAPI,
  schema.d.ts, README and docs/backend/field-types.md updated

modules/cabana/security_coverage_test.go

@@ -43,6 +43,7 @@ var phase09Routes = []adminRoute{
 	{key: "POST /account/avatar"},
 	{key: "DELETE /account/avatar"},
 	{key: "GET /navigation"},
+	{key: "GET /field-types"},
 	{key: "GET /settings"},
 	{key: "GET /settings/{code}/schema"},
 	{key: "GET /settings/{code}"},