Document that VAAPI HEVC on Polaris can't beat already-efficient H.264 (YouTube/
Twitch/stream archives), so output comes out larger and lands in hevc_failed.txt.
Add already_failed() guard so the batch skips known-bad files on queue rebuilds
instead of re-attempting them. Also: MIN_FREE_GB note (start-only check) and a
source-bitrate triage snippet for picking real encode candidates.