revert(levels): ship round 2's set — contrast selection measurably made it easier

Round 3 tested one hypothesis from round 2's data: levels the agent rated 3
carried 59% more 反差 than the rest, so select for it. With both rounds now at
~100 runs, the answer is in and it is no. The back band went from 2.39 ± 0.10 to
2.19 ± 0.08, roughly two standard errors apart, and the levels averaging 2.5 or
better fell from 11 to 6. Contrast is real and it does correlate with being
rated harder; selecting on it does not produce harder levels.

The low retest reliability does not rescue the change either — r = 0.55 limits
ranking one level against another, not the mean of eighteen levels measured a
hundred times.

So round 2's levels are the ones handed over, rebuilt from its runs via
round-set --emitCampaign. select-50 cannot reproduce them: the pool it drew
from has been rebuilt since, so the same flags now return different levels. The
runs are the record.

Rule bot on the shipped set: 50/50 cleared, dead taps 0.00, blind rotations
0.12, worst blind streak 1. 198 tests pass.

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

scripts/build-review-50.mjs

@@ -14,7 +14,7 @@ import { layout } from '../src/geometry.js';
 import { metrics } from '../src/bench.js';
 
 const args = Object.fromEntries(process.argv.slice(2).map(a => { const [k, ...v] = a.replace(/^--/, '').split('='); return [k, v.length ? v.join('=') : true]; }));
-const AGENT = args.agent ?? 'R3 · grok xhigh';
+const AGENT = args.agent ?? 'R2 · grok xhigh';
 const OUT = resolve(args.out ?? 'docs/review-50.html');
 
 const raw = JSON.parse(readFileSync(resolve('levels/campaign-50.json'), 'utf8'));