Batch managed-server log delivery before MainActor publication

[acari-git]

Problem

ManagedServerOutputCoordinator.begin currently creates a Task { @MainActor ... } for every output line. AppViewModel.handleManagedServerOutput then calls appendLog for each line, which rebuilds LogViewState. Under bursty model output this can create unnecessary MainActor work.

Current verification (2026-08-15)

• LogStateCoordinator.append(contentsOf:) supports batch updates.
• Managed server output still reaches it one line at a time.
• LogStateCoordinatorTests pass.

Desired behavior

• Buffer managed-server output off the MainActor.
• Publish bounded batches (for example every 50–100 ms or by batch size).
• Preserve line order, bounded history, fault detection, and final output on process termination.
• Add a burst regression test that verifies MainActor publication count is substantially lower than line count.