docs(experiments): plain-language notes and clearer tables

[leo-gan]

Summary

• New Experiments site page in everyday words: why we ran each test, an example, the trade-off, and the data we timed.
• All 13 titles and stories rewritten in experiment.yaml (the settings file). The Dashboard reads them automatically.
• Experiment tables now match the rest of the Dashboard: green/red cells, `15.9 (1.2×)` vs the fastest row, trials, spread (std), 3 significant digits.
• The old Group / Similar / Close / Time/size front labels are now Vs fastest: Fastest, About the same, A bit slower, Clearly slower, plus Good trade-off. These are not simply Winner / Loser — two libraries can be too close to call.

Validation

• Analysis tests: 85 passed
• All 13 `experiment.yaml` files parse
• Browser (Chrome): story, sample JSON, `3.89 (1×)` / `7.86 (2×)`, trials `92 of 100`, group labels

Notes

• Compare libraries inside one language.