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/template.go

@@ -0,0 +1,109 @@
+package response
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// Slot returns the name of a whole-value placeholder, or empty for constants.
+// Slots preserve the value's JSON type; they are not string interpolation.
+func Slot(value any) string {
+	s, ok := value.(string)
+	if ok && strings.HasPrefix(s, "${") && strings.HasSuffix(s, "}") {
+		return s[2 : len(s)-1]
+	}
+	return ""
+}
+
+func validateTemplate(value any, depth int) error {
+	if depth > 24 {
+		return fmt.Errorf("response: body nesting exceeds 24 levels")
+	}
+	switch v := value.(type) {
+	case map[string]any:
+		for key, item := range v {
+			if strings.TrimSpace(key) == "" {
+				return fmt.Errorf("response: body keys must not be empty")
+			}
+			if err := validateTemplate(item, depth+1); err != nil {
+				return err
+			}
+		}
+	case []any:
+		for _, item := range v {
+			if err := validateTemplate(item, depth+1); err != nil {
+				return err
+			}
+		}
+	case string:
+		if strings.HasPrefix(v, "${") {
+			switch Slot(v) {
+			case "code", "message", "data", "requestId", "ok", "status":
+			default:
+				return fmt.Errorf("response: unknown body slot %q", v)
+			}
+		}
+	case nil, bool, json.Number, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, float32, float64:
+		if _, err := json.Marshal(v); err != nil {
+			return fmt.Errorf("response: invalid body constant: %w", err)
+		}
+	default:
+		return fmt.Errorf("response: body accepts JSON values, got %T", value)
+	}
+	return nil
+}
+
+func cloneTemplate(value any, depth int) any {
+	// Invalid cyclic Go configuration must still reach validation, not overflow.
+	if depth > 24 {
+		return value
+	}
+	switch v := value.(type) {
+	case map[string]any:
+		copy := make(map[string]any, len(v))
+		for key, item := range v {
+			copy[key] = cloneTemplate(item, depth+1)
+		}
+		return copy
+	case []any:
+		copy := make([]any, len(v))
+		for i, item := range v {
+			copy[i] = cloneTemplate(item, depth+1)
+		}
+		return copy
+	default:
+		return value
+	}
+}
+
+func renderTemplate(value any, slots map[string]any, dataMode string) any {
+	if name := Slot(value); name != "" {
+		return slots[name]
+	}
+	switch v := value.(type) {
+	case map[string]any:
+		result := make(map[string]any, len(v))
+		for key, item := range v {
+			slot := Slot(item)
+			if slot == "requestId" && slots[slot] == "" || slot == "data" && slots[slot] == nil && dataMode != "always" {
+				continue
+			}
+			result[key] = renderTemplate(item, slots, dataMode)
+		}
+		return result
+	case []any:
+		result := make([]any, len(v))
+		for i, item := range v {
+			result[i] = renderTemplate(item, slots, dataMode)
+		}
+		return result
+	case string:
+		if strings.HasPrefix(v, "$${") {
+			return v[1:]
+		}
+		return v
+	default:
+		return value
+	}
+}