Greptile Summary

This PR refactors the monolithic Download method in the concurrent downloader by extracting its internal phases into dedicated methods (initMirrorStatus, setupNetwork, getWorkerMirrors, setupTasks, startHelpers, runBalancer, runCompletionMonitor, runHealthMonitor, executeWorkers, handlePause, syncFile). A companion test file covers the extracted helpers.

• The refactoring fixes a nil-pointer risk in runCompletionMonitor by guarding d.State.Downloaded.Load() behind a d.State != nil check that the original inline goroutine lacked.
• executeWorkers now deduplicates and joins multiple worker errors with errors.Join instead of silently overwriting with the last error, surfacing all unique failure causes to callers.
• The bitmap initialization order (InitBitmap \u2192 setupTasks/RestoreBitmap) is preserved correctly, and the pause-at-completion-boundary path now explicitly returns d.syncFile(outFile) rather than falling through to the cancel check.

Confidence Score: 5/5

Safe to merge; the extracted helpers preserve the original control flow and the only finding is a redundant test.

The decomposition is mechanically faithful to the original logic, the defer ordering is correct, the nil-guard improvement is sound, and the new tests cover the key extracted paths. No functional regressions were identified.

No files require special attention.

Important Files Changed

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
internal/engine/concurrent/downloader_helpers_test.go:12-35
**Duplicate test scenario**

`TestHandlePause_CompletionBoundary` (lines 12–35) and `TestHandlePause_CompletionFinalization` (lines 183–213) are structurally identical: both construct a fresh `ConcurrentDownloader` with an empty queue and assert that `handlePause` returns `nil` and leaves the state unpaused. Running the same setup twice adds noise and maintenance overhead without covering any additional behavior — if the logic under test changes, both tests will break (or pass) in lockstep. One of them should be removed.

_{Reviews (3): Last reviewed commit: "Merge branch 'main' into speed-limit-hnd..." | Re-trigger Greptile}