Update specification to include long cast validation rules (#206)

I'm not sure if a text length of `0` is allowed but that was my best
guess :)

docs/SPECIFICATION.md

@@ -303,14 +303,15 @@ A CastAddBody in a message `m` is valid only if it passes these validations:
 
 1. `m.signature_scheme` must be `SIGNATURE_SCHEME_ED25519`.
 2. `m.data.type` must be `MESSAGE_TYPE_CAST_ADD`.
-3. `m.data.body.type` must be `CastAddBody`.
+3. `m.data.body` must be `CastAddBody` type.
 4. `m.data.body.embeds_deprecated` can contain up to 2 valid UTF8 strings whose lengths are >=1 byte and <= 256 bytes if the timestamp is <= 73612800 (5/3/23 00:00 UTC).
 5. `m.data.body.mentions` must contain between 0 and 10 256-bit integer values.
 6. `m.data.body.parent`, if present, must be a valid CastId or a UTF8 string whose length is >= 1 byte and <= 256 bytes.
-7. `m.data.body.text` must contain <= 320 bytes and be a valid UTF8 string.
-8. `m.data.body.mentions_positions` must have unique integers between 0 and length of `text` inclusive.
-9. `m.data.body.mentions_positions` integers must be in ascending order and must have as many elements as `mentions`.
-10. `m.data.body.embeds` can contain up to 2 embeds, each of which is a CastId or valid UTF8 string whose length is >=1 byte and <= 256bytes.
+7. `m.data.body.text` must contain <= 1024 bytes and be a valid UTF8 string.
+8. `m.data.body.type` must be either `CastType.CAST` for casts with text length of 0 <= length <= 320 and `CastType.LONG_CAST` for casts that are 321 <= length <= 1024
+9. `m.data.body.mentions_positions` must have unique integers between 0 and length of `text` inclusive.
+10. `m.data.body.mentions_positions` integers must be in ascending order and must have as many elements as `mentions`.
+11. `m.data.body.embeds` can contain up to 2 embeds, each of which is a CastId or valid UTF8 string whose length is >=1 byte and <= 256bytes.
 
 A CastRemoveBody in a message `m` is valid only if it passes these validations: