Create blank series by default

Separate explicit Traffic preset loading from Add series. The first add creates two empty editable series, with Forecast enabled only after all inputs parse.

README.md

@@ -12,7 +12,7 @@ license: other
 
 A private browser interface for Fracast-0.
 
-The page automatically downloads the 113 KB W8 checkpoint and records the loading time. It starts with an empty series workspace. The preset selector loads deterministic synthetic Traffic data; the source note states that it is a synthetic example rather than a public dataset extract. Each channel receives its own compact forecast chart.
+The page automatically downloads the 113 KB W8 checkpoint and records the loading time. It starts with an empty series workspace. `Add series` creates blank inputs: the first click creates two series, then each click adds one. The preset selector loads deterministic synthetic Traffic data; the source note states that it is a synthetic example rather than a public dataset extract. Each channel receives its own compact forecast chart.
 
 The browser runtime uses the checkpoint's native 48-step head. Each series card has its own 8-48 step horizon slider and trims the native forecast; it does not perform recursive long-horizon rollout. The Traffic preset reserves the selected number of final observations as held-out ground truth. Hand-edited inputs have no held-out truth. The activity log records checkpoint loading, input series count, input lengths, forecast time, throughput, and selected horizons.
 

index.html

@@ -6,7 +6,7 @@
     <meta name="color-scheme" content="light">
     <link rel="icon" href="data:,">
     <title>Fracast-0 Playground</title>
-    <script type="module" crossorigin src="./fracast.D5jki3pT.js"></script>
+    <script type="module" crossorigin src="./fracast.DpKYujZ5.js"></script>
     <link rel="stylesheet" crossorigin href="./assets/index.DRUfRDea.css">
   </head>
   <body>