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

docs/database/models.md

@@ -69,7 +69,7 @@ if errors.As(err, &typeErr) {
 // invalid value for views
 ```
 
-`lagoon.Fill` matches keys by the GORM column name, not the Go field name. A number decoded with `json.Decoder.UseNumber` fills integer and float fields; a fraction or an overflow for an integer field is a `lagoon.FillTypeError`.
+`lagoon.Fill` matches keys by the GORM column name, not the Go field name. A number decoded with `json.Decoder.UseNumber` fills integer and float fields; a fraction or an overflow for an integer field is a `lagoon.FillTypeError`. A string fills a date or time column: an RFC 3339 timestamp such as `2026-10-02T12:30:00Z` fills a `time.Time` or `*time.Time` field, `2026-10-02` a `lagoon.Date` and `14:30:00` a `lagoon.TimeOfDay`, pointer variants included, so a model needs no date type of its own. A string that does not parse is a `lagoon.FillTypeError`. See [Date and time columns](casts-and-validation.md#date-and-time-columns).
 
 ## Creating a record