feat: add typed error message templates and stable replay snapshots

errors/errors.go

@@ -6,7 +6,8 @@ import (
 )
 
 // APIError is the public error shape used by handlers and middleware.
-// Cause is retained for logging and unwrapping, but is never serialized.
+// Cause is retained for logging and unwrapping; default response encoders and
+// replay snapshots never serialize it.
 type APIError struct {
 	Status       int
 	Code         string
@@ -14,6 +15,8 @@ type APIError struct {
 	Cause        error
 	BusinessCode any // Optional public code resolved from a project catalog.
 	reference    bool
+	params       map[string]any
+	paramErr     error
 }
 
 func (e *APIError) Error() string {