[Operational Issue]: tvOSSimulator_Shortstack_x64 build timeout — 2 occurrences across pipelines

## Summary

The `tvOSSimulator_Shortstack_x64` vertical times out during the runtime build step, producing no error output. The build log is truncated mid-execution. This has occurred twice in 2 days across both pipelines.

## Occurrences

| Build | Pipeline | Branch | Duration | Date |
|-------|----------|--------|----------|------|
| [2927799](https://dev.azure.com/dnceng/internal/_build/results?buildId=2927799) | Internal (def 1330) | main | Unknown (agent killed) | Mar 16, 2026 |
| [1339186](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1339186) | Public (def 278) | main | 6h37m | Mar 17, 2026 |

## Details (build 1339186)

- **Job:** tvOSSimulator_Shortstack_x64
- **Start:** 14:10 UTC → **Finish:** 20:47 UTC (6h37m)
- **Log:** Truncated while building `runtime` (`TargetRid=tvossimulator-x64`). No error messages, no exit code — log simply stops.
- **Cascade:** OSX_arm64 phase failed as a dependency.

## Impact

Blocks the public rolling build on main (0% rolling pass rate for 2+ days). The Mar 16 rolling failed due to wix5 GenerateCurrentVersion (#5482); this is a different root cause.

## Possible Causes

- Job-level timeout exceeded (6h+ for a shortstack build is abnormally long)
- Agent resource exhaustion on macOS runner
- Runtime build for tvossimulator-x64 has regressed in build time