Verify normalized task approval facts and preserve signed source types

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a140c4-354a-7823-8f0d-8f717397f786

verifier/internal/sign/sign.go

@@ -4,10 +4,12 @@
 package sign
 
 import (
+	"bytes"
 	"crypto/ed25519"
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"io"
 
 	"github.com/netkingdom/access-engine/pkg/api"
 )
@@ -74,3 +76,20 @@ func Verify(envelope api.DecisionEnvelope, keys map[string]ed25519.PublicKey) er
 	}
 	return nil
 }
+
+// DecodeEnvelope preserves JSON numeric kinds/lexemes emitted by task inputs.
+// Legacy producer envelopes remain byte-compatible. It does not invent another
+// signature canonicalization: CanonicalBytes still calls typed json.Marshal.
+func DecodeEnvelope(raw []byte) (api.DecisionEnvelope, error) {
+	var envelope api.DecisionEnvelope
+	decoder := json.NewDecoder(bytes.NewReader(raw))
+	decoder.UseNumber()
+	decoder.DisallowUnknownFields()
+	if err := decoder.Decode(&envelope); err != nil {
+		return envelope, err
+	}
+	if _, err := decoder.Token(); err != io.EOF {
+		return api.DecisionEnvelope{}, fmt.Errorf("trailing envelope data")
+	}
+	return envelope, nil
+}