feat: judge golden trace and variance stats

[igmarin]

Why

ROADMAP item 4. Judge scores had no golden fixture and no way to talk about inter-run spread. Deltas are not product truth without that.

What

• test/fixtures/judge_traces/canonical.json — 5 canonical dimensions, total 80
• Judge::Response must keep parsing that fixture
• Judge::Variance.call(totals:) — sample mean, stddev, spread (n < 2 → nils)
• Verdict formula unchanged
• ROADMAP: items 2–3 marked shipped; item 4 first slice

Proof

bundle exec ruby -Itest test/judge/variance_test.rb
# 2 runs, 0 failures

bundle exec ruby -Itest test/judge/golden_trace_test.rb
# 1 run, 0 failures

Not in this PR: live --repeat recording of judge totals.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between b0bdef7 and 333aad0.

📒 Files selected for processing (8)

• CHANGELOG.md
• ROADMAP.md
• lib/skill_bench.rb
• lib/skill_bench/judge/README.md
• lib/skill_bench/judge/variance.rb
• test/fixtures/judge_traces/canonical.json
• test/judge/golden_trace_test.rb
• test/judge/variance_test.rb

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