Add correction proposal models

internal/framework/proposals/policy.go

@@ -0,0 +1,47 @@
+package proposals
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// ReplacementPolicy controls how original_text spans are replaced in a target segment.
+type ReplacementPolicy string
+
+const (
+	ReplacementPolicyRequireUnique ReplacementPolicy = "require_unique"
+	ReplacementPolicyReplaceAll    ReplacementPolicy = "replace_all"
+)
+
+func (p ReplacementPolicy) IsValid() bool {
+	switch p {
+	case ReplacementPolicyRequireUnique, ReplacementPolicyReplaceAll:
+		return true
+	default:
+		return false
+	}
+}
+
+func ParseReplacementPolicy(raw string) (ReplacementPolicy, error) {
+	parsed := ReplacementPolicy(strings.TrimSpace(raw))
+	if !parsed.IsValid() {
+		return "", fmt.Errorf("replacement policy must be one of: require_unique, replace_all")
+	}
+	return parsed, nil
+}
+
+func (p *ReplacementPolicy) UnmarshalJSON(data []byte) error {
+	var raw string
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return fmt.Errorf("replacement policy must be a JSON string: %w", err)
+	}
+
+	parsed, err := ParseReplacementPolicy(raw)
+	if err != nil {
+		return err
+	}
+
+	*p = parsed
+	return nil
+}