feat(ui): double-click a step to start the picture there; double-click a screen for what happens on it

- StepsView / StepNode: a double-click on any step with a symbol re-anchors the Steps picture on it (the panel's Start here) — an endpoint or another screen drawn as a boundary opens as its own chapter in one gesture; detected in the view's click path (two clicks on one box within 400 ms) since the flow canvas does not reliably pass dblclick on, with ondblclick kept
- ScreensView / ScreenNode: a double-click on a screen (or an origin) opens its Steps picture
- a boundary's panel says it is not entered instead of "nothing leaves this step"; tooltips and the boundary notes mention the gesture
- spec §3.13, CHANGELOG

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01REFyW9hmNrxhwN5wxRoAkC

ui/src/components/screens/ScreenNode.svelte

@@ -25,6 +25,8 @@
       selected: boolean;
       dimmed: boolean;
       onSelect: (id: string) => void;
+      /** Open what happens from this screen (the Steps picture) — a double-click. */
+      onOpen?: (id: string) => void;
     }
   );
   const layout = $derived(node.layout);
@@ -54,12 +56,13 @@
   class:unreached={info.unreached}
   style={`width:${layout.width}px;height:${layout.height}px`}
   onclick={() => node.onSelect(info.id)}
+  ondblclick={() => node.onOpen?.(info.id)}
   aria-pressed={node.selected}
-  title={info.origin
+  title={(info.origin
     ? `${info.label} — navigates, but no screen reaches it within the walk. In ${info.sub}.`
     : `${info.label} — rendered by ${info.sub}${info.entry ? '. The entry screen.' : ''}${
         info.unreached ? '. No transition in the graph reaches it from the entry.' : ''
-      }`}
+      }`) + (node.onOpen ? ' Double-click for what happens here.' : '')}
 >
   <span class="name">{#if info.entry}<span class="mark" aria-hidden="true">●</span>{/if}{info.label}</span>
   <span class="sub">{info.sub}</span>