feat(scripts): license-filtered motion corpus scraper (#838, t2m-v2 Slice A)

Offline dev tool assembling a permissively-licensed animated-humanoid
corpus for the text-to-motion v2 epic (#837):

- Sketchfab Data API discovery (animated + downloadable), client-side
  license filter by exact label/slug (CC0 / CC Attribution only — the
  search API omits slugs and "CC Attribution" must not prefix-match the
  NonCommercial/ShareAlike/NoDerivs variants); downloads via
  SKETCHFAB_API_TOKEN, discovery-only rows recorded without it.
- OpenGameArt scraping with the per-page License(s) block as the
  authoritative filter (every listed license must be CC0/CC-BY),
  3D-Art check, attachment-only downloads with zip extraction.
- Quaternius CC0 packs enumerated live via the GitHub tree API of the
  mirror repo (hardcoded pack URLs rot — fetch-training-rigs.sh lesson),
  Patreon-Exclusive folders excluded, one manifest row per pack.
- Validation with the editor's own loader: `qtmesh info --json` keeps
  only assets with a skeleton and >= 1 skeletal animation
  (--keep-static opts out).
- Incremental provenance manifest (merged across runs, deduped by
  source_url) + generated ATTRIBUTION.md for CC-BY credits.

Live-tested: 16 assets (15 Quaternius packs + 1 CC-BY OGA rig with 46
bones / 11 animations); validation correctly dropped signs, posed
statues and unanimated base meshes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: fernandotonon