Function register_streaming_functions 

pub fn register_streaming_functions(ctx: &SessionContext)

Registers LaminarDB streaming UDFs with a session context.

Registers the following scalar functions:

• tumble(timestamp, interval) — tumbling window start
• hop(timestamp, slide, size) — hopping window start
• session(timestamp, gap) — session window pass-through
• watermark() — current watermark (returns NULL, no live source)

Use register_streaming_functions_with_watermark to provide a live watermark source from Ring 0.