Add regression coverage for background playback and silent switch behavior

[zachyzissou]

Problem

Background playback and silent/ringer behavior were recently reported broken, then fixed. This needs durable regression coverage so it does not slip again.

Acceptance criteria

• Document required audio session category/options and background modes.
• Add unit or integration coverage for the playback audio session configuration.
• Add a manual TestFlight smoke checklist for lock screen, app backgrounding, silent switch, route changes, and interruptions.
• Verify Now Playing metadata and remote controls still work after background/lock transitions.

Agent execution contract

Readiness: agent:senior-required
Lane: Audio regression

Start in these files:

• OffScript/PlaybackController.swift
• OffScript/NowPlayingPublisher.swift
• OffScriptTests/OffScriptTests.swift
• docs/*

Required verification:

• xcodebuild test -project OffScript.xcodeproj -scheme OffScript -destination 'platform=iOS Simulator,OS=latest,name=iPhone 17 Pro' -only-testing:OffScriptTests CODE_SIGNING_ALLOWED=NO
• Real-device/TestFlight lock screen, silent switch, route-change checklist.

Non-goals / guardrails:

• Do not claim background audio fixed without real-device/TestFlight smoke.

PR expectation: Audio regression PR with unit config proof plus manual checklist.

Low-thinking agents must follow docs/AGENT_EXECUTION_RUNBOOK.md. If the touched code falls outside the files above, stop and tighten the issue or ask for senior review before continuing.

[zachyzissou]

Status: needs real-device validation

Can never close from simulator-only work. Acceptance bullets require an actual iPhone with a signed-in iCloud account / real audio session interruption / lock-screen interaction. Recommend marking this with a 'real-device-required' status so it's clear this isn't pickup-able by a low-thinking agent regardless of available time.

Discovered while reviewing the kanban post-session 2026-04-30.