Fix remote-asset crashes and parallelize hot paths (validate, pack, dedup, sync)

[CaioWing]

Correctness:

• vp fsck no longer aborts on cloud-backed assets: resolved_path raises for
  remote URIs, so byte checks are skipped for them (referential checks still
  apply); regression tests added.
• vp validate no longer floods cloud-referenced datasets with false
  image.unreadable errors: remote assets skip the local header probe.

Performance:

• validate: image header probes now fan out across a thread pool (order
  preserved, report deterministic) instead of decoding sequentially.
• near-duplicate LSH: identical hashes are collapsed before bucketing, so
  batches of re-encoded near-uniform images no longer trigger the O(n^2)
  bucket pathology; over-threshold pairs are marked seen so shared bands
  never re-XOR them. phash backfill is parallel.
• vp eval: each prediction's IoUs are computed once and replayed across all
  10 COCO thresholds (and the precision/recall pass) instead of once per
  threshold.
• WebDataset pack: shards are written in parallel (they're independent tar
  streams), the asset->annotation map is materialized once up front, and
  sample labels serialize via orjson.
• Archive pack: zstd multi-threaded compression (threads=-1) for the
  single-stream tar.zst.
• SQLite index: WAL + synchronous=NORMAL on the live db; snapshot freeze and
  archive pack run a truncating checkpoint first so the copied/hashed
  index.db stays self-contained. Frozen snapshot dbs are left untouched.
• sync ingest pool: results are consumed as they complete (no head-of-line
  blocking on a slow object) and folded in input order, keeping the index
  and summaries deterministic.
• local listing: os.scandir walk captures stat in the same pass as
  discovery (one syscall per file instead of two).

Co-Authored-By: Claude Fable 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01WtJWqB5e95s9FgEZF2VccN