feat(cabana): widget action payload and data channel (quick-261006-eyj)

- pact.AdminActionInput.Payload (json.RawMessage) carries the widget's own
  JSON value untouched; pact.AdminActionResult.Data is passed through as data
- cabana decodes payload with a 64 KiB cap (422 on body), refuses it on the
  toolbar and record routes, and embeds Data once encoded with a 256 KiB cap
  (opaque 500 when larger or unencodable); fill stays filtered
- root .swaggo overrides json.RawMessage so swag keeps record_id and values;
  admin.json and schema.d.ts regenerated (payload?: unknown, data?: unknown)
- TestWidgetPayloadAndData covers pass-through, cap, refusal and data 500
- cabana and pact READMEs, partials-and-widgets and admin-spa docs updated

modules/cabana/phase101_actions_test.go

@@ -135,6 +135,29 @@ func (c actController) AdminActions() []pact.AdminAction {
 		Name: "lookup", Label: "acme.demo::lang.gadgets.lookup", Permissions: []string{"acme.demo.run"},
 		Run: func(_ context.Context, in pact.AdminActionInput) (pact.AdminActionResult, error) {
 			c.spy.record(in)
+			if len(in.Payload) > 0 {
+				// The payload round trip: echo the decoded payload as data
+				// next to a fill that must still be filtered; "big" and
+				// "nan" answer data the framework must reject.
+				var payload any
+				if err := json.Unmarshal(in.Payload, &payload); err != nil {
+					return pact.AdminActionResult{}, err
+				}
+				switch payload {
+				case "big":
+					return pact.AdminActionResult{Data: strings.Repeat("x", 256<<10+1)}, nil
+				case "nan":
+					return pact.AdminActionResult{Data: math.NaN()}, nil
+				}
+				return pact.AdminActionResult{
+					Message: "acme.demo::lang.gadgets.looked_up",
+					Fill:    map[string]any{"name": "reordered", "tenant": "other", "active": []string{"a"}},
+					Data: map[string]any{
+						"echo":  payload,
+						"items": []map[string]any{{"id": 1, "title": "a"}},
+					},
+				}, nil
+			}
 			switch in.Values["name"] {
 			case "invalid":
 				return pact.AdminActionResult{}, &cabana.ValidationError{Details: map[string]any{"name": []string{"Name is taken."}}}