docs: normalize book metrics pages into subsections

[BraCR10]

Summary

Restructures book/src/metrics/*.md: each metric now follows a consistent What it is /
Source / How to read it subsection shape instead of long unbroken prose paragraphs.
Keeps the fuller grounding (definition, interpretation, exact Nostr event/tag source)
but easier to scan.

Test plan

• mdbook build succeeds with no warnings

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📥 Commits

Reviewing files that changed from the base of the PR and between d2fb59d and 98e9566.

📒 Files selected for processing (6)

• book/src/metrics/README.md
• book/src/metrics/activity-grid.md
• book/src/metrics/context-signals.md
• book/src/metrics/disputes-bond-policy.md
• book/src/metrics/longevity-liveness.md
• book/src/metrics/trade-size-consistency.md

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