feat: show rebuilding hash spinner during cross-session resume

fix: use multicore parallel hashing for .tmp resume rebuild

feat: cross-session resume from existing .tmp files

fix: add 120s read timeout to detect stalled downloads

fix: spinner label is customizable, not hardcoded "Verifying"

feat: show braille spinner during remote folder deletion

fix: smart per-folder deletion — no extra API calls, skip tainted paths

fix: remove redundant re-check in folder deletion

fix: verify folders are empty before deleting, top-down

feat: increase max concurrent downloads from 4 to 6

docs: document resumable downloads via HTTP Range headers

feat: resume downloads on retry instead of restarting from scratch

fix: speed/ETA calculation excludes verified skips

docs: document multicore hash verification and parallel pipeline

fix: release inflight bytes during hash verification