feat(telemetry): scaffold fresh-node sync diagnostics (WP-A0)

Adds the anchors the sync-diagnostics signals attach to, with no signals
emitted yet:

- New "Ledger Sync Health" dashboard (uid ledger-sync-health) with the
  standard template-variable block copied from an existing board, plus
  empty "Bootstrap (Domain 0)" and "Sync pipeline" rows.
- Signal index section in the data-collection reference, an operator-flow
  stub in the telemetry runbook, and a glossary anchor.
- A sync_diagnostics group in expected_metrics.json and a matching
  assertion helper in validate_telemetry.py so CI fails when a signal
  regresses to absent.

Also registers the new dashboard uid with the harness so the board is
covered by validation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

OpenTelemetryPlan/09-data-collection-reference.md

@@ -1379,3 +1379,27 @@ prefix=xrpld
 | `trace_consensus`    | `1`     | `consensus.*` spans          |
 | `trace_ledger`       | `1`     | `ledger.*` spans             |
 | `trace_peer`         | `1`     | `peer.*` spans (high volume) |
+
+---
+
+## Fresh-node sync diagnostics
+
+Signals that explain why a freshly-started node is slow to reach, or never
+reaches, a validated ledger (`server_state=full`). Two groups: pre-quorum
+bootstrap (DNS, dial, handshake, UNL/quorum, clock skew) and the post-peering
+ledger/tx-set acquire pipeline.
+
+Rendered by the **Ledger Sync Health** dashboard
+(uid `ledger-sync-health`, rows `Bootstrap (Domain 0)` and `Sync pipeline`).
+Operator flow: [telemetry-runbook.md](../docs/telemetry-runbook.md)
+"Diagnosing slow/stuck fresh sync". Terms:
+[telemetry-glossary.md](../docs/telemetry-glossary.md)
+"Fresh-node sync diagnostics".
+
+The table below is the single index for these signals; one row is added per
+signal as it lands. `Type` is the instrument kind (counter / gauge / histogram /
+span / span attr), `Emit site` the owning source file, and `Panel` the dashboard
+panel that renders it.
+
+| Signal | Type | Emit site | Panel | Meaning |
+| ------ | ---- | --------- | ----- | ------- |