Configuration system: YAML loading and validation

[tac0turtle]

Summary

Implement the configuration system (config/) for loading, validating, and providing typed access to apex's YAML configuration.

YAML over TOML — aligns with Evolve ecosystem and DevOps conventions. People are more accustomed to YAML.

Requirements

• Load from config.yaml (configurable path via --config flag)
• Validate required fields, reject unknown keys
• Sensible defaults for optional fields
• Environment variable overrides for sensitive values (e.g., APEX_AUTH_TOKEN)

Config structure

namespaces:
  eden: "000000000000000000000000000000000000000000000000000000006564656e"
  xo: "00000000000000000000000000000000000000000000000000000000786f"
  collect: "000000000000000000000000000000000000000000000000636f6c6c656374"

data_source:
  type: celestia-node
  endpoint: ws://localhost:26658
  auth_token: ""

storage:
  path: ./data/indexer.db

rpc:
  address: 0.0.0.0:26659

grpc:
  address: 0.0.0.0:26660

sync:
  start_height: 1000000
  backfill_batch_size: 100
  backfill_concurrency: 4

observability:
  metrics_address: 0.0.0.0:9090
  log_level: info
  tracing_enabled: false
  tracing_endpoint: ""

Implementation

• Use gopkg.in/yaml.v3
• Typed Go structs in config/config.go with yaml struct tags
• Validation in config/load.go
• Namespace hex strings decoded and validated (29 bytes)
• Unit tests for loading, defaults, validation errors

References

• Design doc: test_plan.md — Configuration section