kernel HTTP client: empty-close responses surface as 'no header/body separator'

4 of 100 driven requests per run (deterministic across runs — same 4 correlations) fail with transport error `no header/body separator`: the kernel's minimal HTTP/1.1 client (`deja-kernel/src/main.rs:350-366`, `read_to_end` then scan for CRLFCRLF) read zero bytes before the router closed the connection.

Since PR #33 the reason is named on the diff row and in the scorecard warnings, so the class is countable. Needs: characterize why the router closes without writing on exactly these requests (request size? body limit? keep-alive interaction), and make the kernel client robust (retry-once on empty-close, or switch to a real client).