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>

README.md

@@ -1,21 +1,36 @@
 # clocktime
 
-clocktime provides a type that holds a calendar-independent time of day value in 24-hour format.
+Time-of-day and ISO 8601 duration types for APIs, GraphQL, and SQL.
 
-clocktime provides basic types that extend Go's built-in time.Time to provide time-of-day and duration support.
+## Install
 
-## `clocktime.ClockTime
+```bash
+go get gitea.auvem.com/go-toolkit/clocktime
+```
 
-Holds calendar-independent time-of-day in 24-hour format.
+## ClockTime
 
-- Converts to and from `HH:MM:SS` format (subset of ISO8061)
-- Marshals to `[]byte` containing string `HH:MM:SS` for SQL
-- Marshals to `string` in format `HH:MM:SS` for JSON & gqlgen
+```go
+ct, err := clocktime.ParseClockTime("09:30:00")
+t := ct.Time() // 1970-01-01 UTC anchor
+```
 
-## `clocktime.Duration`
+- Strict `HH:MM:SS` parsing via [ParseClockTime]
+- SQL `TIME` columns: stored as `[]byte` / string; scanned from `time.Time`, `[]byte`, or `string`
+- Optional fields: use `*ClockTime`; `IsZero()` is true only for nil pointers
+- Midnight `00:00:00` is valid; use `IsMidnight()` to detect it
 
-Wraps time.Duration with prioritized ISO8061 support and opinionated marshalling.
+## ISODuration
 
-- Converts to and from ISO8061 format (utilises [sosodev/duration](https://github.com/sosodev/duration))
-- Marshals to `uint64` with nanosecond precision. Largest representatable duration is about 290 years, limited by underlying time.Duration type.
-- Marshals to `string` in ISO8061 format for JSON & gqlgen
+```go
+d, err := clocktime.ParseISODuration("PT1H30M")
+ns := d.Duration()
+```
+
+- JSON/GQL: ISO 8601 strings
+- SQL: `int64` nanoseconds (strings accepted on scan)
+- Calendar durations (`P1M`, `P1Y`) convert to approximate nanoseconds and may not round-trip
+
+## gqlgen
+
+Both types implement `MarshalGQL` / `UnmarshalGQL`.