Function encode_into 

pub fn encode_into(env: &ObsEnvelope, out: &mut Vec<u8>)

Encode env into out, length-prefixed. Reuses out’s capacity so caller can amortise allocations across envelopes within one flush window.

Internally builds a fresh SizeCache per call. Prefer encode_into_with_cache on batched paths so the cache amortises across envelopes in the same flush window.