Stellar contract observability/metrics event standard

[truthixify]

Tier: M (2-4 days) Type: feature

Context

Off-chain dashboards need stable metric events from contracts (count of payments, total volume, error counts). Standardize the metric event format so a single indexer can consume across all Wraith Stellar contracts.

Scope

• Define WraithMetricEvent { contract, metric_name, value, dimensions } schema
• Emit from each contract at meaningful points (post-send, post-register, post-claim)
• Off-chain reference indexer in stellar/scripts/metrics-indexer/
• Prometheus exporter format
• Demo dashboard config

Acceptance criteria

• Event standard documented in stellar/METRICS.md
• At least 3 contracts emitting standard events
• Reference indexer working against futurenet
• Demo Prometheus + Grafana dashboard

Files to start with

• All stellar/*/src/lib.rs
• New: stellar/METRICS.md, stellar/scripts/metrics-indexer/