Fix Compare size labels and experiment explainer copy

[leo-gan]

Summary

• Compare Payload size group label is compounded data sizes.
• Compounded choices are both sizes · 100 items and both sizes · 1000 items. The old “every item count” row is gone.
• Why experiments… stays closed until you open it, and no longer fills the screen on arrival.
• That explainer is about queues (1P1C vs 4P4C, 256 B vs 4 KiB), not serializers.
• Experiment 13 story no longer says the suite names five data types.

Validation

• Dashboard unit tests: 22 passed.
• Experiment catalog re-synced from the updated Experiment 13 yaml.

Notes

• Dashboard and experiment copy only. No new timing.