feat: implement Stellar Route Insights Exporter

[mftee]

Summary

• Implements a standalone Stellar Route Insights Exporter under src/exporters/routes/stellar/
• Aggregates per-route metrics and recommendation insights from the bridging system
• Supports three common export formats: JSON, CSV, and Prometheus text exposition

Changes

• src/exporters/routes/stellar/index.ts — Full exporter implementation including:
  • RouteMetric and RecommendationInsight type definitions
  • StellarRouteInsightsExporter class with ingestion, snapshot, and export methods
  • toJSON() / toJSONObject() — structured JSON export
  • toCSV(options) — configurable CSV export for routes and/or insights (delimiter, header, dataset selection)
  • toPrometheus() — Prometheus text exposition with per-route gauges and recommendation sub-scores
  • export(format, options) — unified entry point for all formats
  • stellarRouteInsights singleton export for shared app-wide usage

Export Formats

Format	Method	Description
JSON	toJSON() / toJSONObject()	Full snapshot with routes + insights
CSV	toCSV({ dataset, delimiter })	Routes, insights, or both; configurable delimiter
Prometheus	toPrometheus()	Per-route gauges + recommendation sub-scores

Test plan

• CI build passes (pnpm run build)
• registerRoutes() and registerInsights() correctly upsert entries
• toJSON() returns a valid, parseable snapshot
• toCSV({ dataset: 'routes' }) produces correct headers and rows
• toCSV({ dataset: 'insights' }) produces correct headers and rows
• toCSV({ dataset: 'both' }) combines both datasets
• toPrometheus() renders valid Prometheus text format with correct labels
• export('json' | 'csv' | 'prometheus') delegates to the correct method
• clear() resets all tracked data

closes #542