One ballot per matchup + rating-aware matchmaking; register Opus 4.8 as pending

Every (voter, case, pair) now gets exactly one vote: the client sampler
only draws matchups the browser hasn't voted on (with a graduation state
once all are spent, counter shows voted/total pairs), and the worker
normalizes pair order on insert under a unique (ip, case, pair) index so
side-swapping can't double-count. Daily IP cap drops 300 -> 200 since a
voter graduates at ~171 pairs today.

Matchmaking is now active rather than uniform: unvoted pairs are drawn
with weight closeness/(1+battles) — close ratings and under-played
matchups surface more, unrated combos get placement matches first — with
a 25% pure-exploration floor. Ratings come from the community pool (60s
cached) and fall back to local votes. Rankings stay Bradley-Terry.

Also registers the empty outputs/claude-opus-4-8/ folder as a pending
registry entry.

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

README.md

@@ -33,7 +33,7 @@ NAGI STUDIO 的 LLM 测评案例集：同一段提示词，不同「模型 × Ha
 | Kimi K2.6 | Moonshot AI | Kimi Code（本地） | Thinking | 02 |
 | MiniMax M3 | MiniMax | MiniMax Code 网页版 | Thinking | 01 |
 
-待测：Mistral Medium 3.5 · GLM-5.1 · Qwen3.7-Max（欢迎 PR 补充）
+待测：Claude Opus 4.8 · Mistral Medium 3.5 · GLM-5.1 · Qwen3.7-Max（欢迎 PR 补充）
 
 > 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。