fix(downloads): prevent removed items from returning

[ShiroKSH]

Summary

• prevent late progress events from recreating a download that was removed from the authoritative queue
• preserve a cancelled queue item when a worker reports completion concurrently
• cover removal and re-queue state transitions

Fixes #274.

Checks

• pnpm test — 71 passed
• pnpm check — 0 errors
• cargo fmt --all -- --check
• cargo test --workspace — 702 passed

cargo clippy --workspace --all-targets -- -D warnings remains blocked by 37 pre-existing findings outside this change.