Adds a third layered streaming watchdog (FIRST_CONTENT_TIMEOUT=90s) in
parse_sse_stream: aborts if no content delta / tool-use block arrives
within 90s of body-parse start, even while Anthropic keepalive `ping`
events keep the 120s idle guard happy. Catches the connect->headers->
ping-dribble-forever freeze the send-header (v0.2.5) and idle-read
(v0.2.4) guards both miss.

Also refactors send_with_header_timeout -> send_with_header_timeout_dur
(injectable Duration) and adds a black-hole unit test proving the
send-header guard fires within budget (accept-never-respond TcpListener
-> StreamRead in 0.68s).

Adapter version 0.2.5 -> 0.2.6. Syncs memories + tree descriptions.