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/state/viewModel.ts

@@ -71,6 +71,7 @@ export interface StoryTreeNodeView {
 
 export interface StoryView {
   currentProse: string | null;
+  atBootIntro: boolean;
   choices: StoryChoiceView[];
   tree: StoryTreeNodeView[];
 }
@@ -215,8 +216,14 @@ export function toView(state: GameState, content: GameContent): GameView {
   const availableChoices = getAvailableChoices(state, content);
   const allChoices = node?.choices ?? [];
 
+  const bootEntryNodeId = content.storyNodes.find((n) =>
+    n.triggers?.some((t) => t.type === 'boot'),
+  )?.id;
+  const atBootIntro = node != null && node.id === bootEntryNodeId;
+
   const story: StoryView = {
     currentProse: node?.prose ?? null,
+    atBootIntro,
     choices: allChoices.map((choice) => {
       const available = availableChoices.some((c) => c.id === choice.id);
       return {