clone batch UI: keep active item and completed counts consistent during generation

[PowerBeef]

Clone batch generation should present internally consistent progress while work is in flight.

Source report: #22

Problem

The clone batch UI can show contradictory progress state during active generation, which makes it hard to trust the batch status display.

Evidence

• April 18, 2026 validation report: Dev validation report: April 18, 2026 #22
• Manual validation observed: Generating item 2/2... 0 of 2 clips completed · Item 1 active
• The batch eventually finished successfully, so the defect is in progress/state presentation rather than basic completion

Current ownership

• Sources/Services/BatchGenerationRunner.swift
• Sources/Views/Components/BatchGenerationSheet.swift
• QwenVoiceTests/BatchGenerationRunnerTests.swift

Acceptance

During clone batch generation, the visible active item, completed count, and overall item index remain consistent from first item through completion.

Focused rerun

• python3 scripts/harness.py validate
• Scoped Computer Use pass for the clone batch flow