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/__tests__/viewModel.test.ts

@@ -256,6 +256,55 @@ describe('action columns projection', () => {
   });
 });
 
+describe('story.atBootIntro', () => {
+  it('is true at the boot-entry node and false after entering a different node', () => {
+    const base = buildContent({
+      resources: [{ id: 'coin', name: 'Coin', startAmount: 0 }],
+      actions: [
+        {
+          id: 'forage',
+          name: 'Forage',
+          group: DEFAULT_GROUP,
+          durationMs: 1000,
+          yields: [{ resourceId: 'coin', amount: 1 }],
+        },
+      ],
+    });
+    const story = buildStoryContent(
+      [
+        {
+          id: 'boot_intro',
+          prose: 'Boot.',
+          triggers: [{ type: 'boot', targetNodeId: 'boot_intro' }],
+        },
+        {
+          id: 'fork_choice',
+          prose: 'Which way?',
+          choices: [
+            {
+              id: 'pick_a',
+              label: 'High road',
+              outcomes: [{ type: 'setFlag', flag: 'route_a' }],
+              targetNodeId: 'route_a_beat',
+            },
+          ],
+        },
+        { id: 'route_a_beat', prose: 'The high road.' },
+      ],
+      base.actionsById,
+      base.resourcesById,
+    );
+    const c = { ...base, ...story };
+    const state = createGameState(c);
+    // Simulate boot: enter the boot-entry node
+    enterStoryNode(state, c, 'boot_intro');
+    expect(toView(state, c).story.atBootIntro).toBe(true);
+    // After moving past boot into fork_choice, atBootIntro must be false
+    enterStoryNode(state, c, 'fork_choice');
+    expect(toView(state, c).story.atBootIntro).toBe(false);
+  });
+});
+
 describe('story tree projection', () => {
   it('builds a tree marking seen and active nodes', () => {
     const state = createGameState(content);