flow: structured dtypes, and the widgets that read them

A series, record or list message declares its shape instead of riding
DType.JSON, so a widget binds a shape rather than some JSON and a wrong
binding is refused before anything runs. A list declares its item type,
which is what keeps list[float] expressible for a pipeline.

On top of that: an agenda over a list, a notification over a record, and
a dashboard alert channel that publishes engine faults as one — so a
panel can show what went wrong without a flow wiring it by hand.

Also: only None means a node published nothing, a falsy value of the
wrong shape is now the named error it always should have been; and the
gauge's readout says its size is viewBox geometry rather than type scale.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/components/Flow/NodePanel.tsx

@@ -50,7 +50,19 @@ import { PANEL_SECTION, PanelTitle, SidePanel } from "./SidePanel"
 
 const NodeEditor = lazy(() => import("./NodeEditor"))
 
-const DTYPES: DType[] = ["float", "int", "str", "bool", "json"]
+const DTYPES: DType[] = [
+  "float",
+  "int",
+  "str",
+  "bool",
+  "json",
+  "series",
+  "record",
+  "list",
+]
+
+/** What a list may hold. One declared level: no list of lists. */
+const ITEM_DTYPES: DType[] = ["record", "float", "int", "str", "bool", "json"]
 
 /** Radix selects cannot hold an empty value, so "no secret" needs a name. */
 const NO_SECRET = "__none__"
@@ -274,6 +286,29 @@ function PortList({
                 ))}
               </SelectContent>
             </Select>
+            {spec.dtype === "list" ? (
+              <Select
+                value={spec.item ?? "record"}
+                onValueChange={(value) =>
+                  update(index, { item: value as DType })
+                }
+              >
+                <SelectTrigger
+                  className="!h-8 w-[92px] text-sm"
+                  aria-label="Item type"
+                  title="What each item of the list is"
+                >
+                  <SelectValue />
+                </SelectTrigger>
+                <SelectContent>
+                  {ITEM_DTYPES.map((dtype) => (
+                    <SelectItem key={dtype} value={dtype}>
+                      {dtype}
+                    </SelectItem>
+                  ))}
+                </SelectContent>
+              </Select>
+            ) : null}
             <Input
               type="number"
               min={0}
@@ -765,6 +800,9 @@ const PLACEHOLDER: Record<DType, string> = {
   bool: "False",
   str: '""',
   json: "{}",
+  series: '{"lines": []}',
+  record: "{}",
+  list: "[]",
 }
 
 const SCAFFOLD_DOC =