docs: design focused NPC portrait dialogue

[JOY (JOY)]

Summary

• Add the focused NPC dialogue portrait and lip-sync design packet for issue Add focused NPC portrait and speaking animation to dialogue UI #139.
• Define alpha tiers: text-timed speaking fallback first, audio-amplitude later, provider viseme/blendshape lip sync only when Convai or another provider plus compatible rigs are ready.
• Link the new design from GitBook nav, systems index, alpha UX flow, AI NPC roadmap, ROADMAP, and Play Mode smoke checklist.

Local reviewer fallback

• Docs-only change reviewed manually against second-spawn-backlog and code-review skill requirements.
• No Unity, Nakama runtime, client, scene, prefab, package, or backend code changed.
• LLM/provider output remains presentation-only for speaking animation and cannot mutate authoritative game state.

Verification

• tools\windows\check-backlog-sync.ps1
• npx.cmd --yes markdownlint-cli2 ROADMAP.md docs/SUMMARY.md docs/design/03-systems-index.md docs/design/30-alpha-ux-flow.md docs/design/37-ai-npc-backend-client-roadmap.md docs/design/56-focused-npc-dialogue-portrait-lipsync-design.md docs/setup/play-mode-smoke-checklist.md
• git diff --check
• rg -n "—|–" ROADMAP.md docs/SUMMARY.md docs/design/03-systems-index.md docs/design/30-alpha-ux-flow.md docs/design/37-ai-npc-backend-client-roadmap.md docs/design/56-focused-npc-dialogue-portrait-lipsync-design.md docs/setup/play-mode-smoke-checklist.md returned no matches.

References:

• Convai Unity lip-sync documentation: https://docs.convai.com/api-docs/plugins-and-integrations/unity-plugin/adding-lip-sync-to-your-character

Relevant to #139 and #262.