fix: strict ClockTime parsing and ISODuration SQL

Validate constructors, strict HH:MM:SS parse, UTC ClockTimeFromTime,
ISODuration Value/Scan, Compare and TextMarshaler, codec.go split.

Co-authored-by: Cursor <cursoragent@cursor.com>

doc.go

@@ -0,0 +1,14 @@
+// Package clocktime provides ClockTime (time-of-day) and ISODuration types with
+// JSON, GraphQL, SQL, and text serialization.
+//
+// # ClockTime
+//
+// [ClockTime] stores HH:MM:SS without a date. Use [ParseClockTime] for strict parsing.
+// Optional fields should use *ClockTime; [ClockTime.IsZero] is true only for nil pointers.
+// Midnight 00:00:00 is valid—use [ClockTime.IsMidnight] to test for midnight.
+//
+// # ISODuration
+//
+// [ISODuration] wraps [time.Duration] with ISO 8601 formatting via sosodev/duration.
+// SQL storage uses int64 nanoseconds; strings are also accepted on scan.
+package clocktime