docs: add AI NPC believability evaluation

[JOY (JOY)]

Summary

• Add docs/design/53-ai-npc-believability-evaluation.md as the AI NPC believability QA contract.
• Update the alpha acceptance matrix, AI NPC roadmap, GitBook summary, and ROADMAP.
• Create and link backlog issue Add AI NPC believability smoke tests and evidence pack #251 for Play Mode and backend smoke tests.

Verification

• git diff --check
• git diff --cached --check
• npx markdownlint-cli2 ROADMAP.md docs/SUMMARY.md docs/design/34-alpha-acceptance-matrix.md docs/design/37-ai-npc-backend-client-roadmap.md docs/design/53-ai-npc-believability-evaluation.md
• rg "[\u2013\u2014]" ROADMAP.md docs\SUMMARY.md docs\design\34-alpha-acceptance-matrix.md docs\design\37-ai-npc-backend-client-roadmap.md docs\design\53-ai-npc-believability-evaluation.md
• Local code-review fallback: approved for docs-only design and QA contract. No Unity, Nakama, package, scene, or provider-key changes.

Closes #251 only when runnable smoke tests land. This PR creates the design contract and backlog tracking.