feat: release configurable API response contracts

Define reusable response bodies in .api with typed code mappings, per-route overrides and matching OpenAPI schemas. Add application and callback encoders, early response policy selection, and isolated remote-consumer release verification.

Release v0.8.0-preview after full race, vet, build and local MySQL workflow acceptance.

response/encoder.go

@@ -0,0 +1,60 @@
+package response
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// Result is the transport-independent input to an Encoder. Err is available for
+// application error mapping; default encoders never expose it or its causes.
+type Result struct {
+	Status                   int
+	Code, Message, RequestID string
+	Data                     any
+	Err                      error
+}
+
+// Payload is a complete response. Encoders must be safe for concurrent use.
+// Status and headers are intentionally part of the contract: callback protocols
+// may acknowledge an error using a different HTTP status or media type.
+type Payload struct {
+	Status int
+	Header http.Header
+	Body   []byte
+}
+
+type Encoder interface{ Encode(Result) (Payload, error) }
+type EncoderFunc func(Result) (Payload, error)
+
+func (f EncoderFunc) Encode(r Result) (Payload, error) { return f(r) }
+
+func JSON(status int, value any) (Payload, error) {
+	data, err := json.Marshal(value)
+	if err != nil {
+		return Payload{}, err
+	}
+	return Payload{Status: status, Header: http.Header{"Content-Type": {"application/json; charset=utf-8"}}, Body: append(data, '\n')}, nil
+}
+
+func Text(status int, value string) Payload {
+	return Payload{Status: status, Header: http.Header{"Content-Type": {"text/plain; charset=utf-8"}}, Body: []byte(value)}
+}
+
+// Default is immutable; applications can use different encoders in one process.
+var defaultEncoder = MustNew(Config{})
+
+func Default() Encoder { return defaultEncoder }
+
+// RawJSON leaves successful data unwrapped. Errors use the selected error
+// policy; callback protocols with custom acknowledgments should use EncoderFunc.
+func RawJSON(errors Encoder) Encoder {
+	if errors == nil {
+		errors = Default()
+	}
+	return EncoderFunc(func(r Result) (Payload, error) {
+		if r.Err != nil || r.Status >= 400 {
+			return errors.Encode(r)
+		}
+		return JSON(r.Status, r.Data)
+	})
+}