Add candidate artifact codec decoding

internal/modules/dnd/codec/npcoccurrences/codec.go

@@ -22,6 +22,7 @@ const (
 var schemaAssets embed.FS
 
 var _ contracts.ArtifactCodec[dnd.NPCOccurrenceList] = (*Codec)(nil)
+var _ contracts.CandidateArtifactCodec[dnd.NPCOccurrenceList] = (*Codec)(nil)
 
 type Codec struct{}
 

internal/modules/dnd/codec/npcoccurrences/codec_test.go

@@ -106,6 +106,39 @@ func TestCodecSupportsEmptyListAndPreservesCollectionPresenceInCandidates(t *tes
 	}
 }
 
+func TestCodecCandidateDecodeOwnsValues(t *testing.T) {
+	codec := New()
+	input := validList()
+	content, err := codec.EncodeCandidate(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	first, err := codec.DecodeCandidate(content)
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := codec.DecodeCandidate(content)
+	if err != nil {
+		t.Fatal(err)
+	}
+	nameOffset := bytes.Index(content, []byte("Mira Thorn"))
+	if nameOffset < 0 {
+		t.Fatal("candidate JSON does not contain NPC name")
+	}
+	content[nameOffset] = 'X'
+	if first.Occurrences[0].Name != "Mira Thorn" {
+		t.Fatal("DecodeCandidate() retained input bytes")
+	}
+	first.Occurrences[0].Name = "changed"
+	first.Occurrences[0].SourceRefs[0].SourceID = "changed"
+	if input.Occurrences[0].Name != "Mira Thorn" || input.Occurrences[0].SourceRefs[0].SourceID != "session-alpha" {
+		t.Fatal("DecodeCandidate() retained input values")
+	}
+	if second.Occurrences[0].Name != "Mira Thorn" || second.Occurrences[0].SourceRefs[0].SourceID != "session-alpha" {
+		t.Fatal("DecodeCandidate() returned aliased values")
+	}
+}
+
 func TestCodecStrictlyRejectsMalformedUnknownAndTrailingJSON(t *testing.T) {
 	validJSON := `{"occurrences":[{"npc_id":"npc:test-mira","name":"Mira Thorn","kind":"dialogue","source_refs":[{"source_id":"session","start_unit_id":1,"end_unit_id":1}]}]}`
 	for _, test := range []struct{ name, raw, want string }{