feat(12.2-01): add lagoon.Date and lagoon.TimeOfDay with Fill and required support

- Date (DATE) and TimeOfDay (TIME) with Scanner, Valuer, JSON and text forms
- Fill falls back to encoding.TextUnmarshaler for string sources after
  every existing conversion, so time.Time and the new types fill from JSON
- required treats a zero time.Time, Date or TimeOfDay as empty
- lagoon README, models and casts-and-validation docs

modules/lagoon/fill.go

@@ -2,6 +2,7 @@ package lagoon
 
 import (
 	"database/sql"
+	"encoding"
 	"encoding/json"
 	"fmt"
 	"log/slog"
@@ -184,7 +185,52 @@ func convertValue(src reflect.Value, destType reflect.Type) (reflect.Value, erro
 	if src.Type().ConvertibleTo(destType) {
 		return src.Convert(destType), nil
 	}
-	return reflect.Value{}, fmt.Errorf("cannot assign %s to %s", src.Type(), destType)
+	assignErr := fmt.Errorf("cannot assign %s to %s", src.Type(), destType)
+	if out, handled, err := convertText(src, destType); handled {
+		if err != nil {
+			return reflect.Value{}, fmt.Errorf("%w: %w", assignErr, err)
+		}
+		return out, nil
+	}
+	return reflect.Value{}, assignErr
+}
+
+var (
+	textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
+	scannerType         = reflect.TypeOf((*sql.Scanner)(nil)).Elem()
+	dateType            = reflect.TypeOf(Date{})
+	timeOfDayType       = reflect.TypeOf(TimeOfDay{})
+)
+
+// convertText fills a destination type whose pointer implements
+// encoding.TextUnmarshaler (time.Time, Date, TimeOfDay) from a string or
+// []byte source, so a JSON date string fills a date column. It runs only
+// after every other conversion failed, and it skips types that also
+// implement sql.Scanner (other than Date and TimeOfDay), which setField
+// already fills through Scan, so nothing that filled before changes path.
+// handled is false when the fallback does not apply.
+func convertText(src reflect.Value, destType reflect.Type) (reflect.Value, bool, error) {
+	var text []byte
+	switch v := src.Interface().(type) {
+	case string:
+		text = []byte(v)
+	case []byte:
+		text = v
+	default:
+		return reflect.Value{}, false, nil
+	}
+	ptrType := reflect.PointerTo(destType)
+	if !ptrType.Implements(textUnmarshalerType) {
+		return reflect.Value{}, false, nil
+	}
+	if ptrType.Implements(scannerType) && destType != dateType && destType != timeOfDayType {
+		return reflect.Value{}, false, nil
+	}
+	ptr := reflect.New(destType)
+	if err := ptr.Interface().(encoding.TextUnmarshaler).UnmarshalText(text); err != nil {
+		return reflect.Value{}, true, err
+	}
+	return ptr.Elem(), true, nil
 }
 
 // convertNumber parses a json.Number, which a request decoder using