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/StoryView.tsx

@@ -3,19 +3,18 @@ import { useGameStore } from '../state/store';
 import { StoryProseLog } from './StoryProseLog';
 import { StoryTree } from './StoryTree';
 
+const SHELL_HEIGHT = 'h-[calc(100dvh-6rem)]';
+
 export function StoryView() {
   const tree = useGameStore((s) => s.story.tree);
   const log = useGameStore((s) => s.storyLog);
   const selectedId = useGameStore((s) => s.selectedStoryNodeId);
   const currentProse = useGameStore((s) => s.story.currentProse);
-  const choicesLength = useGameStore((s) => s.story.choices.length);
-
-  // Boot intro: show Continue button when there's prose, no choices, and no tree yet
-  const isBootIntro = currentProse != null && choicesLength === 0 && tree.length === 0;
+  const isBootIntro = useGameStore((s) => s.story.atBootIntro);
 
   if (isBootIntro) {
     return (
-      <div className="flex h-[calc(100dvh-6rem)] flex-col items-center justify-center gap-6">
+      <div className={`flex ${SHELL_HEIGHT} flex-col items-center justify-center gap-6`}>
         <p className="max-w-lg text-center text-lg leading-relaxed text-slate-100 whitespace-pre-wrap">
           {currentProse}
         </p>
@@ -31,11 +30,11 @@ export function StoryView() {
   }
 
   return (
-    <div className="grid h-[calc(100dvh-6rem)] grid-cols-[3fr_2fr] gap-4">
+    <div className={`grid ${SHELL_HEIGHT} grid-cols-[3fr_2fr] gap-4`}>
       <StoryTree
         nodes={tree}
         selectedId={selectedId}
-        onSelect={(id) => useGameStore.getState().setSelectedStoryNodeId(id)}
+        onSelect={(id) => gameRuntime.selectStoryNode(id)}
       />
       <StoryProseLog log={log} selectedId={selectedId} />
     </div>