When the preflight-selected Soulseek folder produces zero usable files —
every transfer failed/aborted/stalled (the Slipknot dead-peer case: all
tracks 'Completed, Aborted' at 0 bytes) — _poll_album_bundle_downloads
returns []. download_album_to_staging used to return that with
fallback=False, so try_dispatch marked the whole batch failed and nothing
was retried elsewhere until the next wishlist run.
Flip that branch to fallback=True so the existing, proven per-track flow
takes over and re-searches every missing track across ALL sources/peers.
This reuses the per-track multi-source robustness instead of reimplementing
candidate-folder retry inside the bundle.
Tests: tests/test_soulseek_album_fallback.py drives the preflight-reuse path
with a stubbed poll — empty poll -> fallback=True (differential-verified it
fails without the fix), healthy poll -> fallback stays False. Downstream
routing (fallback=True -> per-track) already covered by
test_album_bundle_dispatch.py.
