docs: align alpha hub naming

[JOY (JOY)]

Summary

• Align public alpha docs around The Garden, Training Lane, Signal Mast, Repair Row, and Downtown Dayton Recovery Zone naming.
• Mark the older Dayton demo region doc as historical and update GitBook navigation label.
• Update design/backlog references, setup docs, inventory, art, UX, AI NPC, and execution docs so player-facing text no longer presents Relay Yard as the current hub name.

Verification

• npx.cmd --yes markdownlint-cli2 @files on all changed markdown files: pass, 0 errors.
• git diff --check: pass.
• rg -n "—|–" @files: pass, no generated em dash/en dash in changed files.
• rg check for legacy terms outside migration/history docs: pass.
• tools/windows/check-backlog-sync.ps1 -NoGitHub: WARN only for existing broad ROADMAP TODOs without nearby issue refs; tracked files check passed.

Local reviewer fallback

• Docs-only change; no Unity, Nakama, AGENTS.md, or .claude/CLAUDE.md changes.
• No authority, LLM, backend, or runtime behavior changes.
• Naming migration is consistent with docs/design/50-dayton-real-map-slice-design.md and docs/design/51-the-garden-hub-design.md.

Notes

• Updated issue Build Garden roster and Gate prep UI #138 title to Build Garden roster and Gate prep UI to match the docs.