Add Arena blind battles, model-grouped Demo Show, and a community leaderboard

Each case now defaults to Arena mode: two anonymous artifacts (identities
hidden, footers suppressed) battle via A / B / Neither, reveal after the
vote, with the same clip-reveal motion language as the rest of the site.
Demo Show replaces the per-combination tab strip with one tab per model;
multi-harness models get a harness switcher and a side-by-side compare,
so the lineup no longer bloats as harnesses multiply.

Votes live in lib/arena.ts (localStorage, custom event fan-out) and rank
via Bradley-Terry fits on pairwise wins, LMSYS Chatbot Arena style, with
'neither' surfaced as a both-bad rate. The #board section stays locked
until 3 votes are cast in each case (blind first, board after), then
shows your votes plus the ranking. When VITE_ARENA_API is configured the
store syncs to the zero-dependency Cloudflare Worker + D1 backend in
worker/ (Turnstile verification, per-IP daily caps, idempotent retries);
deploy steps live in worker/README.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -37,6 +37,12 @@ NAGI STUDIO 的 LLM 测评案例集：同一段提示词，不同「模型 × Ha
 
 > 此表为人工维护的快照；站点上的「模型阵容」表由 `models/*.json` 实时生成，以站点为准。
 
+## Arena 盲评与社区榜单
+
+- 每个案例默认进入 **Arena 模式**：随机抽两份产出匿名对战（A / B / 都不好），投完票才揭晓身份；右上角可切到 **全部查看**（按模型分组浏览，同一模型多个 Harness 可单看或并排对比）。
+- 在两个案例各投满 03 票后解锁**社区榜单**：按 Bradley-Terry 评分排名（LMSYS Chatbot Arena 同思路），「都不好」单独计入双败。
+- 投票默认存浏览器本地；部署 `worker/`（Cloudflare Worker + D1 + Turnstile 防刷，免费档即可）后自动升级为全站众包数据，步骤见 [worker/README.md](./worker/README.md)。
+
 ## 结构 / Structure
 
 ```