docs: add art look-dev backlog

[JOY (JOY)]

Summary

• add an EPIC-AX art look-dev lane to the alpha production backlog
• create and link Project feat(ai): add model-backed agent decisions #5 issues [Alpha Art] Build Anime-Ready Semi-Real look-dev scene #174-[Alpha Art] Validate candidate body rig and animation registry compatibility #178 for scene, asset shortlist, shader/material, Web/mobile budget, and rig/action validation
• update acceptance, roadmap, changelog, and project tracking docs so art execution is visible next to engineering backlog

Local review

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

Checked manually:

• no Unity scene, prefab, package, backend, or authority logic changes
• no AGENTS.md / CLAUDE.md project-context change needed
• issue links and Project feat(ai): add model-backed agent decisions #5 linkage verified for [Alpha Art] Build Anime-Ready Semi-Real look-dev scene #174
• art lane remains a validation gate, not a final art bible or purchase decision

Verification

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

Issues

• Tracks [Alpha Art] Build Anime-Ready Semi-Real look-dev scene #174
• Tracks [Alpha Art] Shortlist Anime-Ready Semi-Real asset candidates #175
• Tracks [Alpha Art] Validate clean PBR material and shader recipe #176
• Tracks [Alpha Art] Define Web and mobile visual budget for alpha #177
• Tracks [Alpha Art] Validate candidate body rig and animation registry compatibility #178