docs: add alpha asset shortlist

[JOY (JOY)]

Summary

• add docs/design/41-alpha-asset-shortlist.md for issue [Alpha Art] Shortlist Anime-Ready Semi-Real asset candidates #175
• compare Full Anime Cel / Toon versus Anime-Ready Semi-Real PBR after JOY clarified the game should be bright, attractive, and character-forward rather than grimdark
• shortlist player/NPC bodies, enemy/boss candidates, Relay Yard and Gate environment packs, and shader/material candidates with import-test guidance
• link the shortlist from GitBook summary, systems index, roadmap, changelog, and art direction docs

Local review

Local code-review fallback: APPROVED for docs/research-only scope.

Checked manually:

• no Unity scene, prefab, package, backend, or authoritative gameplay changes
• no purchase or import decision is forced
• source links are store pages rather than copied asset text
• issue [Alpha Art] Shortlist Anime-Ready Semi-Real asset candidates #175 acceptance criteria are covered

Verification

• npx.cmd markdownlint-cli2 "docs/**/*.md" "*.md"
• git diff --check
• local markdown link resolver for changed docs
• em-dash scan on changed docs

Closes #175