Common utilities and shared components for Sentinel proxy

This crate provides shared functionality used across all Sentinel components, including observability (metrics, logging, tracing), error types, and common utilities.

Module Organization

• [ids]: Type-safe identifier newtypes (CorrelationId, RequestId, etc.)
• [types]: Common type definitions (ByteSize, Priority, etc.)
• [errors]: Error types and result aliases
• [limits]: Resource limits and rate limiting
• [observability]: Metrics, logging, and tracing
• [circuit_breaker]: Circuit breaker state machine
• [registry]: Generic type-safe registry abstraction