NimBold 4b7c00dae3 docs: add download queue coordinator design spec ...

Backend QueueManager as sole concurrency gatekeeper with Arc<Semaphore>,
uniform permit parking for aria2/media/native, CAS-based lazy semaphore
shrink, idle-park dispatcher, and reactive frontend store.

Design surfaced and fixes four correctness bugs:
- idle CPU spin on empty queue (peek-then-Notify park)
- fetch_sub underflow to usize::MAX (CAS retirement loop)
- aria2 permit leak on early RPC return (permit parking in active_permits)
- gid completion race before aria2_gids store (pending_completion buffer)

2026-06-16 16:48:05 +03:30

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2026-06-16-download-queue-coordinator-design.md

docs: add download queue coordinator design spec

2026-06-16 16:48:05 +03:30