Add CAN traffic observability and vcan integration CI

[s2015-turtle]

Summary

• publish standard diagnostics with categorized CAN TX counters, recognized RX counters, command coalescing, and per-motor feedback rates and ages
• expose lock-free driver and transport metric snapshots backed by atomic counters
• add deterministic unit coverage for transmitted and coalesced frame accounting
• add a Jazzy integration job that exercises both directions through ros2_socketcan and a Linux vcan interface
• document how to interpret measured rates in multi-axis systems without presenting them as total bus utilization

Validation

• git diff --check
• package manifests validated with xmllint
• local ROS builds were intentionally not run; the five-distribution matrix and vcan job provide build and runtime validation

Closes #17