docs/ci: add production DNP3 engineering contract and PR build gate

[masarray]

Summary

Adds repository-level production governance, strengthens the existing DNP3 app-specific agent contract, and adds a Windows/.NET 8 pull-request build gate.

Engineering protections

• root-cause-first workflow with a three-patch circuit breaker
• protocol/UI/service ownership boundaries remain explicit
• expected transport/parser/protocol outcomes use Try/typed Result/status rather than routine exceptions
• infrastructure exceptions are contained at meaningful boundaries
• bounded asynchronous diagnostics with aggregation/rate limiting
• no one-row-per-frame synchronous WPF rendering; batch/coalesce live evidence updates
• deterministic Start/Stop/transport lifecycle and last-known-good state promotion
• evidence integrity: no fabricated values, timestamps, command outcomes, or protocol identity
• measurable latency/queue/allocation/CPU/memory expectations for hot paths

CI

Adds a pull-request Windows build gate for the main Dnp3MasterTester project using .NET 8. The CI build does not package or publish an installer.

The existing app-level Dnp3MasterTester/AGENTS.md remains the closest source authority for DNP3 runtime behavior; the new root contract governs repository-wide engineering and release discipline.