Replace generator-based result collection with explicit per-future
handling. Each future is now processed independently with a 600s timeout.
Before: _results.extend(f.result() for f in _futures)
- One exception stops the generator, remaining results are lost
- No timeout: one hung job blocks the entire tick
After: as_completed() + per-future try/except
- Each future handled independently
- 600s timeout prevents indefinite blocking
- Failed futures are logged and counted as failures
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