Use task instead of backgroundTask for Awaitable CE backing

[Copilot]

backgroundTask always offloads the CE's initial continuation to a thread-pool thread, even when that's unnecessary. Replacing it with task keeps execution on the calling thread, suspending only at genuine async boundaries.

Changes

• AAD.fs/Awaitable.fs (#if TASKS branch): replace backgroundTask with task for the builder binding and both map/bind helpers

// before
let awaitable = backgroundTask
let inline map f (x:Task<_>) = backgroundTask { let! v = x in return f v }
let inline bind f (x:Task<_>) = backgroundTask { let! v = x in return! f v }

// after
let awaitable = task
let inline map f (x:Task<_>) = task { let! v = x in return f v }
let inline bind f (x:Task<_>) = task { let! v = x in return! f v }