feat(12.2-02): add the fileupload field with deferred uploads committed on save

- type: fileupload compiles the D-08 keys and binds to the model's attach.Relation at boot
- X-Session-Key (cabana.SessionKeyHeader) carries the form session key; RecordInput.SessionKey
- GET and POST .../{id}/files/{field}: list with pending uploads, multipart upload into attach.Store
- the create and update save attaches the session's pending files in its transaction
- swagger2openapi folds formData parameters into a multipart requestBody
- admin OpenAPI, TS types, conformance cases, README and forms docs

modules/cabana/contracts.go

@@ -89,6 +89,9 @@ type CompiledController struct {
 	// fields declare, the only ones a request may render with a record id.
 	partials     map[string]*compiledPartial
 	formPartials map[string]bool
+	// files are the form's `type: fileupload` fields bound to the model's
+	// attach.Relation, keyed by field name.
+	files map[string]*compiledFile
 }
 
 // Registry is the immutable controller map keyed by controller ID.