feat: add daemon cleanup stress test and tighten stream foreground handling

[DjDeveloperr]

Summary

• Added a dedicated daemon cleanup stress harness that starts isolated temporary project daemons, exercises health/metrics, and verifies process-group, listener-port, and metadata cleanup.
• Wired the harness into npm run test:stress:daemon and documented it under testing.
• Tightened viewer foreground detection so hidden or offscreen canvases stop reporting as active, reducing unnecessary encoder work.
• Paired that client signal with native session cleanup so the video path pauses frame input when the last subscriber leaves and resumes when streaming restarts.
• Documented the current encoding behavior and the low-footprint interpretation of viewer activity.

Testing

• cargo test --manifest-path server/Cargo.toml
• cargo fmt --manifest-path server/Cargo.toml --check
• npm run --prefix client typecheck
• npx prettier --check ...
• node --check scripts/stress/daemon-cleanup.mjs
• Ran scripts/stress/daemon-cleanup.mjs once against the debug binary with a passing cleanup result