Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/websocket/tcpserver/server_test.go

@@ -36,8 +36,8 @@ func testConf(mutate func(*config.JSCServer)) *config.JSCServer {
 
 // newServer returns a server for conf with its own hub.
 func newServer(conf *config.JSCServer) (*Server, *websocket.Hub) {
-	h := websocket.NewHub(conf.WebSocket, discardLog)
-	return New(conf, h, discardLog), h
+	h := websocket.NewHub(conf.WebSocket, websocket.WithLogger(discardLog))
+	return New(conf, h, WithLogger(discardLog)), h
 }
 
 // startServer serves on a local port, returning the server's hub, its