fix(sender): capture headless virtual display directly

[Betafer]

Summary

• capture the TargetBridge virtual display directly when it is the only online display
• avoid the ScreenCaptureKit shareable-content lookup in the headless monitor-mode path
• preserve the existing physical-display and mirror path
• keep auxiliary audio capture enabled when audio relay is requested

Why

A headless Mac mini can already create the TargetBridge virtual display, but the sender still depended on Screen Recording permission because it tried to discover that display through ScreenCaptureKit. Direct CGDisplayStream capture is sufficient for the virtual display and makes unattended startup reliable without a temporary monitor.

Validation

• all 115 TBDisplaySender Xcode tests pass on macOS
• added focused tests for virtual-only, physical-display, offline-virtual-display, and auxiliary-audio decisions
• verified on Mac mini M4 -> 2017 Intel iMac: HEVC video and 48 kHz audio start with the virtual display while Screen Recording preflight remains denied

This PR intentionally contains only the direct headless capture decision and its tests.