docs: add typed failure and async diagnostics contract

[masarray]

Summary

Strengthens the existing ARIEC61850 agent contract without changing its protocol/provenance architecture.

Added failure architecture

• expected parser/protocol/state-machine outcomes use typed Result/Try contracts instead of routine exceptions
• coherent error taxonomy per protocol layer rather than ad-hoc Result classes
• socket/XML/.NET/infrastructure exceptions are contained at meaningful boundaries and converted
• high-rate GOOSE/SV/receive paths emit compact diagnostics/counters only
• diagnostic queues must be bounded with aggregation/dedup/rate limiting
• diagnostic sink failure must never block protocol processing or corrupt association state
• explicit typed failure/state transition replaces throw/retry-delay workaround patterns

Documentation-only change; runtime protocol behavior is unchanged.