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.en.md

@@ -37,6 +37,12 @@ Pending: Mistral Medium 3.5 · GLM-5.1 · Qwen3.7-Max (PRs welcome)
 
 > This table is a hand-maintained snapshot; the site's registry section is generated live from `models/*.json` and is authoritative.
 
+## Arena blind battles & community board
+
+- Each case defaults to **Arena mode**: two anonymous artifacts battle (A / B / Neither) and identities reveal only after you vote; switch to **Demo Show** to browse everything (tabs are grouped by model — a model with several harnesses can be viewed singly or side by side).
+- Casting 03 votes in each case unlocks the **community leaderboard**, ranked by Bradley-Terry scores (the LMSYS Chatbot Arena approach); "neither" counts separately as both-bad.
+- Votes stay in the browser by default; deploying `worker/` (Cloudflare Worker + D1 + Turnstile, free tier) upgrades the board to sitewide crowdsourced data — see [worker/README.md](./worker/README.md).
+
 ## Structure
 
 ```