Contain CAN receive and diagnostics executor failures

[s2015-turtle]

Summary

• Contain receive, diagnostics, executor and full transmit-thread exceptions; latch observable transport failure and propagate hardware read/write ERROR.
• Publish provider-independent best-effort failure diagnostics, distinguish expected ROS context shutdown, and clean partial startup/failed runs before restarting.
• Join threads before releasing ROS resources, with finite executor idle waits rather than relying on cancel during shutdown.
• Add nine ROS callback/diagnostics/lifecycle regression cases, including standard/unknown exceptions and fallback diagnostics.

Validation

• Independent source review completed on identical tree 6aaac07ecf9974e3e35ad34fd335dce50b7945f5; no blocking findings.
• Local strict C++17 metrics compilation/smoke and git diff --check passed.
• All five ROS distribution build/tests (Humble, Jazzy, Kilted, Lyrical, Rolling) and vcan integration passed on exact head e80209b (run 36904899409, attempt 2). Cancelled dependency-download jobs passed targeted retries.

Limits

This does not bound a publisher or callback that never returns (#31). No thread detachment or physical motor safety guarantee is introduced. Automatic motor re-enable policy (#40) is unchanged.

Closes #33