feat: external opcode metadata for archive sources

[skylenet]

Summary

• Adds opcodes and opcodes_file fields to the archive source config, allowing opcode count data to be provided via an external JSON file when not using EEST fixtures
• Opcode data is written to opcode_count at the top level of each test in summary.json, and the UI opcode heatmap/test detail views now read from this unified field (with fallback to eest.info.opcode_count for backward compatibility)
• Adds archive source documentation to docs/configuration.md and config.example.yaml which were previously missing

Config example

source:
  archive:
    file: https://github.com/.../artifacts/6222084759
    opcodes: "opcodes_tracing.json"
    # Optional: separate archive for the opcodes file
    opcodes_file: https://github.com/.../artifacts/6222074312
    steps:
      test:
        - "perf-devnet-3/testing/*.txt"

Test plan

• go build ./pkg/executor/... ./pkg/config/... compiles
• go test ./pkg/executor/... ./pkg/config/... passes
• golangci-lint run passes on changed packages
• TypeScript type-checks with npx tsc --noEmit
• Manual: run with archive source + opcodes config, verify summary.json contains opcode_count per test
• Manual: verify UI opcode heatmap renders for non-EEST suite