feat: D24 fact intervals (--as-of) and bin/stack assistant helpers. (#37)

Store valid_from/valid_to on leafs and filter search by calendar day without
overloading D16 source staleness; stack start/start-assistant wires brain + PicoClaw.

internal/facts/interval.go

@@ -0,0 +1,44 @@
+package facts
+
+// Interval of truth for a fact leaf (D24 / OQ5). Not D16 source staleness.
+//
+// Empty valid_from and valid_to means "always" (legacy leafs). Empty asOf
+// means "do not filter". Dates compare as YYYY-MM-DD (lexicographic).
+
+// NormalizeDay keeps the calendar day from ISO-8601 or bare dates.
+func NormalizeDay(s string) string {
+	s = trimSpace(s)
+	if len(s) >= 10 && s[4] == '-' && s[7] == '-' {
+		return s[:10]
+	}
+	return s
+}
+
+func trimSpace(s string) string {
+	i, j := 0, len(s)
+	for i < j && (s[i] == ' ' || s[i] == '\t' || s[i] == '\n' || s[i] == '\r') {
+		i++
+	}
+	for j > i && (s[j-1] == ' ' || s[j-1] == '\t' || s[j-1] == '\n' || s[j-1] == '\r') {
+		j--
+	}
+	return s[i:j]
+}
+
+// ActiveAt reports whether a fact with [validFrom, validTo] holds at asOf.
+// validTo empty = open-ended. Both ends inclusive.
+func ActiveAt(validFrom, validTo, asOf string) bool {
+	asOf = NormalizeDay(asOf)
+	if asOf == "" {
+		return true
+	}
+	from := NormalizeDay(validFrom)
+	to := NormalizeDay(validTo)
+	if from != "" && asOf < from {
+		return false
+	}
+	if to != "" && asOf > to {
+		return false
+	}
+	return true
+}