docs: add scoped process-bus failure and diagnostics contract

[masarray]

Summary

Adds src/AGENTS.md as a scoped source addendum while preserving the existing repository-level engineering invariants.

Added source protections

• three-patch root-cause circuit breaker
• explicit Try/typed Result/status handling for expected packet/parser/analysis failures
• infrastructure exception containment outside high-rate loops
• defensive BER/APDU/SV bounds and finite-value validation
• bounded non-blocking async diagnostics with aggregation/rate limiting
• transactional selected-stream coherent snapshot publication
• capture/UI backpressure and no one-render-per-packet behavior
• measurable packet/sample throughput, allocation, queue, CPU/memory and render-latency expectations

The root AGENTS.md remains authoritative for product boundary, selected-stream semantics, licensing and release rules. Documentation-only change.