Speech calls keep one connection per server, opened when listening starts

transcribe and speak share an httpx client per speech server with a 300 s
keep-alive, and run on the engine's loop. stt warms it with GET /v1/models
when an utterance opens, so a reply's first sentence finds the connection
open: a new one over a lossy link cost up to a second (openpanel V4).

docs/reference/node-types.md

@@ -460,7 +460,8 @@ such as one that sends whole clips.
 | `voice` | `af_heart` | one of Kokoro's voices, such as `bm_george` |
 
 Each sentence is one call to `{url}/v1/audio/speech` and published as soon as
-it is spoken. A sentence spoken before in the same voice is reused rather than
+it is spoken. Calls to one server share a connection, which Speech to text opens
+when an utterance starts, so a reply's first sentence does not wait for one. A sentence spoken before in the same voice is reused rather than
 spoken again, for the last 256 sentences. A reference carries `"append": true`
 while the sentences before it are still playing, which queues it on a panel's
 `speaker`. The first sentence of a reply carries none and cuts off whatever