fix: broken fallback to single downloader

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Greptile Summary

This PR fixes two related gaps in the single-threaded download path: DownloadPausedMsg was never emitted when a single-threaded download was paused (the pool now sends it when isPaused && err != nil), and ActiveWorkers was never set or cleared, leaving progress state inconsistent. It also adds rate-limit retry logic (429/503 Retry-After) to the single downloader and wires UpdateChunkStatus into progressReader so the chunk bitmap is populated during single-threaded transfers.

• Pool pause fix (pool.go): The discriminating condition isPaused && err != nil correctly identifies single-threaded paused downloads (which return a non-nil error) vs. concurrent ones (which send the message themselves and return nil).
• Single downloader enhancements (single/downloader.go): Introduces a child context (dlCtx) for pause/cancel, deferred ActiveWorkers lifecycle management, 429/503 retry-after loop, and chunk-bitmap updates via progressReader.flushWithTime.
• State cleanup (types/progress.go): SessionReset now zeroes ActiveWorkers; the noisy UpdateChunkStatus skipped debug log is removed since single-threaded downloads call it with no bitmap.

Confidence Score: 5/5

The core bug fixes are correct and well-scoped; the two findings are minor quality concerns that do not affect correctness of the main fix.

The pause-message fix and ActiveWorkers lifecycle management are correct. The throttledReader context mismatch only causes marginal pause-response delay under rate limiting, and the missing retry tests do not affect current runtime behavior.

internal/engine/single/downloader.go — the rate-limit retry loop and the throttledReader context are the areas worth a second look

Important Files Changed

Filename	Overview
internal/download/pool.go	Adds DownloadPausedMsg emission for single-threaded paused downloads; the discriminating condition (isPaused && err != nil) correctly distinguishes single-threaded (returns non-nil error on pause) from concurrent (returns nil, sends message itself)
internal/engine/single/downloader.go	Adds ActiveWorkers lifecycle, a child context for cancellation, rate-limit retry loop, and UpdateChunkStatus calls to progressReader; throttledReader still uses parent ctx instead of dlCtx, and the retry loop has no test coverage
internal/engine/types/progress.go	SessionReset now zeroes ActiveWorkers and removes a noisy debug log from UpdateChunkStatus
internal/engine/single/downloader_test.go	Adds ActiveWorkers post-completion assertions; does not cover the new rate-limit retry code path
internal/engine/types/progress_test.go	Extends SessionReset test to verify ActiveWorkers is zeroed; straightforward and correct
internal/download/manager.go	Adds debug logging around single-threaded download outcome; minimal, correct change
cmd/root_startup.go	Logs version and commit at startup for easier debugging; safe one-liner addition

_{Reviews (4): Last reviewed commit: "feat: single downloader Retry-After" | Re-trigger Greptile}

Context used:

• Rule used - What: All code changes must include tests for edge... (source)

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Binary Size Analysis

⚠️ Size Increased

Version	Human Readable	Raw Bytes
Main	17.00 MB	17821988
PR	17.00 MB	17826084
Difference	4.00 KB	4096

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❌ Test Failures on windows-latest

• github.com/SurgeDM/Surge/cmd: TestRmClean_Offline_Works