feat(compare): add normalized gain metric

[christso]

Summary

Adds Hake's normalized gain (`g`) to `agentv compare` output, measuring improvement relative to remaining headroom.

The metric

```
g = (score_candidate − score_baseline) / (1 − score_baseline)
```

Raw delta (`Δ`) tells you how much scores changed. Normalized gain tells you how much of the available improvement was captured:

Baseline	Candidate	Δ	g	Interpretation
0.10	0.55	+0.45	0.50	Captured 50% of remaining headroom
0.90	0.95	+0.05	0.50	Same proportional gain, despite smaller Δ
0.50	0.25	−0.25	−0.50	Regression: lost 50% of headroom
1.00	1.00	0.00	null	No headroom, metric undefined

Returns `null` when baseline is already 1.0 (perfect score). Null values are excluded from mean computation.

Where it appears

• Table output: `g: +0.256` in summary line
• Matrix pairwise: `g +0.256` alongside existing `Δ`
• JSON output: `mean_normalized_gain` in summary, `normalized_gain` per matched result

Red/Green E2E

Before (main — no `g`):
```
Summary: 2 wins, 1 loss, 0 ties | Mean Δ: +0.267 | Status: improved
```

After (this branch):
```
Summary: 2 wins, 1 loss, 0 ties | Mean Δ: +0.267 | g: +0.256 | Status: improved
```

JSON output now includes `normalized_gain` per test and `mean_normalized_gain` in summary.

Changes

• `apps/cli/src/commands/compare/index.ts` — `computeNormalizedGain()`, `normalizedGain` on `MatchedResult`, `meanNormalizedGain` in summary, display in `formatTable` and `formatMatrix`
• `apps/cli/test/commands/compare/compare.test.ts` — 10 new tests covering all cases
• `apps/web/src/content/docs/docs/tools/compare.mdx` — docs updated with formula, interpretation table, updated output examples

Test plan

• 50/50 tests pass (10 new tests added)
• Typecheck, lint, build all pass (pre-push hook)
• Red/green CLI e2e verified
• JSON output verified: `normalized_gain` per test, `mean_normalized_gain` in summary
• Docs updated

Closes #1100

🤖 Generated with Claude Code

[cloudflare-workers-and-pages]

Deploying agentv with    Cloudflare Pages

Latest commit:	ea1e60a
Status:	✅  Deploy successful!
Preview URL:	https://38a450aa.agentv.pages.dev
Branch Preview URL:	https://feat-1100-normalized-gain.agentv.pages.dev

View logs