fix(ui): restore boot-intro continue and fix story tree a11y

Finding 1 (critical): atBootIntro was derived from tree.length === 0 in
StoryView.tsx, but buildStoryTree always returns all topology-root nodes,
so tree.length is always > 0 and the Continue button never rendered.
Fix: add atBootIntro: boolean to StoryView (viewModel), computed by
finding the boot-trigger node id without hardcoding the literal string,
then comparing to the current node. StoryView.tsx reads it from the store.
Also add a viewModel test covering both true and false cases.

Finding 2: onSelect in StoryView.tsx called useGameStore.getState()
directly, bypassing the runtime command layer. Fix: add
GameRuntime.selectStoryNode(id) and wire onSelect through gameRuntime.

Finding 3: aria-pressed on story tree nodes wrongly signals toggle-button
semantics. Fix: replace with aria-current={isSelected ? 'true' : undefined}.

Optional: hoist h-[calc(100dvh-6rem)] to SHELL_HEIGHT const in StoryView.tsx.

src/ui/StoryTree.tsx

@@ -36,7 +36,7 @@ function StoryTreeNode({ node, depth, selectedId, onSelect }: StoryTreeNodeProps
         className={labelClasses}
         style={{ marginLeft: `${depth * 1}rem` }}
         onClick={() => onSelect(node.id)}
-        aria-pressed={isSelected}
+        aria-current={isSelected ? 'true' : undefined}
       >
         <span className="flex items-center gap-1.5">
           {node.active && (