Add 5K extended display mode

[DrDavidL]

Summary

• rebase onto the updated main streaming stack and keep the upstream direct-capture, cursor, TCP latency, and backpressure changes intact
• add a sender source selector for Extended display vs Mirror MacBook while using the new main capture pipeline
• send the selected capture source in the hello payload so the receiver status can show what is being streamed
• update sender copy, screen-recording permission text, and English docs for the expanded stream profiles

Validation

• make in TargetBridge-Receiver/TBReceiverC
• xcodebuild -scheme TBDisplaySender -configuration Debug -derivedDataPath TargetBridge-Sender/.build/DerivedData CODE_SIGN_IDENTITY= CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=NO build
• git diff --check origin/main...HEAD

Notes

• The app build script also compiled successfully, but its final Desktop copy step is blocked in this sandbox, so validation used direct xcodebuild for a clean exit.
• The generated TBDisplaySenderBuildInfo.swift build stamp is intentionally excluded from this PR.

[swellweb]

Great work David! The Extended display / Mirror MacBook picker is exactly what was missing. The code is clean and well integrated on the current main. The waitForMirrorDisplay() approach is solid. Looking forward to merging this once you mark it ready!