Lightning v3.1 - choose day-first or month-first date reading

You can now choose how all-numeric dates such as 12/02/2025 are read out. Set the new optional date_format parameter on Lightning v3.1 requests:

`date_format`	Order	`12/02/2025` reads as
`EU`	Day first (`DD/MM`)	12 February 2025
`US`	Month first (`MM/DD`)	December 2, 2025
omitted	Unchanged. English reads month first	December 2, 2025

date_format is accepted on the dedicated /waves/v1/lightning-v3.1/* endpoints and on the unified /waves/v1/tts route, over HTTP, SSE streaming, and WebSocket. Any other value returns 400 with invalid_enum_value (options EU, US). Over WebSocket, the error arrives as a message with status: "error".

Dates with the month written as a word, like 12 February 2025, read the same with any setting.

What changed in the docs:

TTS Best Practices now shows default and EU readings for numeric dates.
The Lightning v3.1 model card no longer recommends DD/MM/YYYY without date_format.