Add D&D NPC extraction and validation

internal/modules/dnd/validate/npcs/diagnostics/diagnostics.go

@@ -0,0 +1,53 @@
+// Package diagnostics provides bounded, safe text for deterministic NPC
+// validator decisions and warnings.
+package diagnostics
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+const (
+	MaxIssues         = 20
+	MaxDisplayedRunes = 128
+	MaxMessageBytes   = 4096
+)
+
+func Truncate(value string) string {
+	runes := []rune(value)
+	if len(runes) <= MaxDisplayedRunes {
+		return value
+	}
+	return string(runes[:MaxDisplayedRunes-1]) + "…"
+}
+
+func Quote(value string) string { return strconv.Quote(Truncate(value)) }
+
+func Aggregate(prefix string, issues []string) string {
+	displayed := make([]string, 0, min(len(issues), MaxIssues))
+	for len(displayed) < len(issues) && len(displayed) < MaxIssues {
+		issue := Truncate(issues[len(displayed)])
+		candidate := aggregateMessage(prefix, append(displayed, issue), len(issues)-len(displayed)-1)
+		if len([]byte(candidate)) > MaxMessageBytes {
+			break
+		}
+		displayed = append(displayed, issue)
+	}
+	return aggregateMessage(prefix, displayed, len(issues)-len(displayed))
+}
+
+func aggregateMessage(prefix string, issues []string, omitted int) string {
+	message := prefix + ": " + strings.Join(issues, ", ")
+	if omitted > 0 {
+		message += fmt.Sprintf("; %d additional issue(s) omitted", omitted)
+	}
+	return message
+}
+
+func min(left, right int) int {
+	if left < right {
+		return left
+	}
+	return right
+}