edge: add integration tests, synthetic publisher, and devnet guide

[armcconnell]

Part 4 of 4 — edge feed parser stack

1. edge: add Top-of-Book parser framework #3522 — edge: add Top-of-Book parser framework ← start here
2. edge: add JSON/CSV/Unix-socket output sinks #3523 — edge: add JSON/CSV/Unix-socket output sinks
3. edge: add feed runner, manager, HTTP API, and CLI #3524 — edge: add feed runner, manager, HTTP API, and CLI
4. this PR — edge: add integration tests, synthetic publisher, and devnet guide

Draft until the prior three land.

Summary

• Integration tests exercise parser + sink end-to-end against synthetic wire bytes: normal refdata-then-marketdata ordering, cold-start buffering where marketdata arrives first, the buffer cap + most-recent-wins overwrite semantics, CSV output, and JSON output.
• testutil/publisher.go is a programmable synthetic Top-of-Book publisher that builds raw wire-format frames (header + messages) for deterministic tests, with no real feed needed.
• testutil/cmd/topofbook-publisher/ wraps the publisher as a CLI tool that streams frames onto a multicast group — useful for local devnet testing where a real producer isn't available.
• DEVNET_TEST.md walks through bringing the edge feature up end-to-end in the local devnet: containers, device setup, multicast group creation with subscriber allowlist, running the synthetic publisher on one client, enabling the edge feed on another, and verifying records flow through.

Testing Verification

• Integration tests run in go test with no special infrastructure.
• DEVNET_TEST.md was dogfooded during development — any developer can follow it end-to-end to reproduce the feature locally.