feat(streaming): hedge slow demand-position article fetches on a second connection

[javi11]

Summary

• Cherry-pick of the hedge-fetch feature merged as feat(streaming): hedge slow demand-position article fetches on a second connection #938, targeting main directly (previously merged into docs/update-nzb-streaming-bench-results, so it currently rides along in docs(bench): refresh nzb-streaming-benchmarks results against main #933; this gives it its own PR against main).
• Once a demand-position article fetch runs past max(400ms, 2x rolling median) without a byte, races a second BodyStreamPriority fetch on another connection and takes whichever completes first, cancelling the loser.

Test plan

• Cherry-picked cleanly from merged commit fc3b42a
• Existing hedge_test.go covers hedging behavior
• CI green