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/http.go

@@ -22,7 +22,7 @@ func (h *Hub) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		CompressionMode: mode,
 	})
 	if err != nil {
-		h.log.Debug("upgrade failed", "error", err)
+		h.logger().Debug("upgrade failed", "error", err)
 		return // Accept has already written the HTTP error.
 	}
 	defer conn.CloseNow() //nolint:errcheck // finished with; it may already be closed
@@ -39,7 +39,7 @@ func (h *Hub) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 	defer h.unregister(c)
 
-	h.log.Info("client connected", "client", c.id, "ip", ip)
+	h.logger().Info("client connected", "client", c.id, "ip", ip)
 	c.run(ctx)
-	h.log.Info("client disconnected", "client", c.id, "ip", ip)
+	h.logger().Info("client disconnected", "client", c.id, "ip", ip)
 }