feat(encode): write the TOML 1.1 output form

Assisted-by: DeepSeek V4.1 Flash

datetime.go

@@ -28,29 +28,42 @@ type LocalDate struct{ time.Time }
 type LocalTime struct{ time.Time }
 
 // String returns the TOML-canonical rendering of the local date-time, e.g.
-// "1979-05-27T07:32:00" or "...:00.000000123" when the time has a fractional
-// second. The fractional component is zero-padded to nanosecond precision.
+// "1979-05-27T07:32" or "1979-05-27T07:32:00.5" when the time carries a
+// fractional second. TOML 1.1 makes the seconds optional, so they appear only
+// when they are non-zero, and a fraction drops its trailing zeros.
 func (ldt LocalDateTime) String() string {
-	base := ldt.Format("2006-01-02T15:04:05")
-	if ns := ldt.Nanosecond(); ns > 0 {
-		return base + "." + fmt.Sprintf("%09d", ns)
-	}
-	return base
+	return ldt.Format("2006-01-02T") + clockString(ldt.Time)
 }
 
 // String returns the TOML-canonical rendering of the local date, e.g.
 // "1979-05-27".
 func (ld LocalDate) String() string { return ld.Format("2006-01-02") }
 
-// String returns the TOML-canonical rendering of the local time, e.g.
-// "07:32:00" or "...:00.000000123" when the time has a fractional second.
-// The fractional component is zero-padded to nanosecond precision.
-func (lt LocalTime) String() string {
-	base := lt.Format("15:04:05")
-	if ns := lt.Nanosecond(); ns > 0 {
-		return base + "." + fmt.Sprintf("%09d", ns)
+// String returns the TOML-canonical rendering of the local time, e.g. "07:32"
+// or "07:32:00.5" when the time carries a fractional second.
+func (lt LocalTime) String() string { return clockString(lt.Time) }
+
+// clockString renders a time of day the way TOML writes it: the seconds appear
+// only when the value carries them, and a fractional second drops its trailing
+// zeros, so half a second is "00.5" and not "00.500000000". Both are the same
+// value either way; the shorter form is the one TOML 1.1 allows.
+func clockString(t time.Time) string {
+	out := t.Format("15:04")
+	ns := t.Nanosecond()
+	if t.Second() != 0 || ns != 0 {
+		out += t.Format(":05")
 	}
-	return base
+	if ns > 0 {
+		out += "." + strings.TrimRight(fmt.Sprintf("%09d", ns), "0")
+	}
+	return out
+}
+
+// offsetString renders an offset date-time, the fourth TOML kind, in the same
+// shape: no zero seconds, no trailing zeros in the fraction, and the offset
+// written as "Z" when it is zero.
+func offsetString(t time.Time) string {
+	return t.Format("2006-01-02T") + clockString(t) + t.Format("Z07:00")
 }
 
 var (