fix: keep paired tab updates live after runtime terminal fallback (#19022)

* fix: preserve session publication during runtime terminal fallback

* refactor(runtime): align fallback epoch comment and test preamble

Match the file's `// Why:` comment convention on the inherited
publication epoch, and drop a redundant duplicate mocks import in the
lineage regression test while keeping the required side-effect order.

No behavior change.

src/main/runtime/orca-runtime-create-runtime-owned-mobile-session-terminal.ts

@@ -120,7 +120,8 @@ export class OrcaRuntimeWithCreateRuntimeOwnedMobileSessionTerminal extends Orca
     }
     const next: RuntimeMobileSessionTabsSnapshot = {
       worktree: worktreeId,
-      publicationEpoch: `headless:${Date.now().toString(36)}`,
+      // Why: a fresh epoch retires the current publisher, so clients drop its later tab updates.
+      publicationEpoch: existing?.publicationEpoch ?? `headless:${Date.now().toString(36)}`,
       snapshotVersion: (existing?.snapshotVersion ?? 0) + 1,
       // Why: activating the new tab also focuses its group, so a "+" targeting a specific split group makes that group active too.
       activeGroupId:

src/main/runtime/runtime-owned-terminal-publication-lineage.test.ts

@@ -0,0 +1,57 @@
+import { expect, it, vi } from 'vitest'
+import type { RuntimeMobileSessionTabsResult } from '../../shared/runtime-types'
+
+// Fragments stay side-effect ordered: mocks, then lifecycle, then fixtures.
+const { OrcaRuntimeService } = await import('./orca-runtime-test-mocks.spec')
+await import('./orca-runtime-test-lifecycle.spec')
+const { store, TEST_WORKTREE_ID } = await import('./orca-runtime-test-fixtures.spec')
+
+it.each(['renderer:active-generation', 'headless:active-generation'])(
+  'keeps %s live when runtime-owned creation supplements its inventory',
+  async (publicationEpoch) => {
+    const runtime = new OrcaRuntimeService(store)
+    runtime.setPtyController({
+      spawn: vi.fn().mockResolvedValue({ id: 'pty-runtime-fallback' }),
+      write: () => true,
+      kill: () => true,
+      getForegroundProcess: async () => null
+    })
+    runtime.syncWindowGraph(0, {
+      tabs: [],
+      leaves: [],
+      mobileSessionTabs: [
+        {
+          worktree: TEST_WORKTREE_ID,
+          publicationEpoch,
+          snapshotVersion: 7,
+          activeGroupId: null,
+          activeTabId: null,
+          activeTabType: null,
+          tabs: []
+        }
+      ]
+    })
+    const events: RuntimeMobileSessionTabsResult[] = []
+    const unsubscribe = runtime.onMobileSessionTabsChanged(
+      (snapshot) => events.push(snapshot),
+      'paired-client'
+    )
+    try {
+      const created = await runtime.createMobileSessionTerminal(`id:${TEST_WORKTREE_ID}`, {
+        activate: false,
+        select: false,
+        navigation: 'caller',
+        clientNavigationId: 'paired-client'
+      })
+      expect(created.tab.status).toBe('ready')
+      expect(created.publicationEpoch).toBe(publicationEpoch)
+      expect(created.snapshotVersion).toBeGreaterThan(7)
+      expect(events.at(-1)).toMatchObject({
+        publicationEpoch: `${publicationEpoch}:client-navigation`,
+        tabs: [expect.objectContaining({ id: created.tab.id, status: 'ready' })]
+      })
+    } finally {
+      unsubscribe()
+    }
+  }
+)