feat(ledger-close-cadence): measure observed Horizon ledger close spacing

[Caneryy]

Summary

Adds a read-only Ledger Close Cadence Explorer that fetches a bounded Horizon GET /ledgers?order=desc&limit=N sample and measures adjacent close-time intervals.

• Computes median / min / max intervals from validated UTC timestamps
• Flags missing sequence numbers separately from unusually long intervals
• Skips malformed records without discarding the rest of the sample
• Surfaces sample size, first/last ledger, and selected network

A long interval in the sample is an observation — not proof of an outage.

Files

Entire slice under features/ledger-close-cadence/ (35 files: scaffold + domain libs, UI, fixtures, MSW, unit/a11y/e2e coverage).

Out of scope

• No live network uptime SLA
• No unbounded historical scan
• No changes outside this feature folder

Test plan

• npm run verify:features -- ledger-close-cadence
• npm test -- --run features/ledger-close-cadence (50 tests)

Closes #469

[drips-wave]

@Caneryy Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits