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

docs/backend/partials-and-widgets.md

@@ -70,7 +70,7 @@ lookup:
 - `action` names an action the controller registers through `pact.HasAdminActions`.
 - `fill` lists the writable scalar fields of the same form the action may write back.
 
-The SPA posts the widget's values to `.../widgets/{field}`. cabana checks the CSRF header, the controller's and the action's permissions and the record scope, then calls the action's `Run` with a `pact.AdminActionInput`. The answer's `pact.AdminActionResult` carries a message and the fill values; keys outside `fill` and values that are not scalars are dropped before the response is written. An action may return a `cabana.ValidationError` to answer 422 on a field.
+The SPA posts `record_id`, the fill snapshot `values` and, when the element supplied one, `payload` to `.../widgets/{field}`. cabana checks the CSRF header, the controller's and the action's permissions and the record scope, then calls the action's `Run` with a `pact.AdminActionInput`. The action reads the payload from the `Payload` field as raw JSON (any value up to 64 KiB, exactly the bytes the client sent, nil when absent) and decodes it itself; it must treat any ids inside the payload as untrusted input and re-check them against its own scope, because the framework never uses the payload to select the record. The answer's `pact.AdminActionResult` carries a message and the fill values; keys outside `fill` and values that are not scalars are dropped before the response is written. It may also carry `Data`, any JSON value up to 256 KiB encoded, which is passed through as `data` and is not filtered like `fill` (a larger or unencodable value is a 500). Toolbar and record actions refuse a payload. An action may return a `cabana.ValidationError` to answer 422 on a field.
 
 ## Toolbar actions