Publish source-health observations as coherent samples #309

🤖 Generated by the Agentic Engineer

Part of #283.

Concurrent source queries and background probes can interleave independent readiness and timestamp gauge updates, leaving a readiness value paired with another completion's observation time.

For publishers, operators and product consumers, the expected result is consistent validation and honest public metadata. The hypothesis is that enforcing the existing boundary at its real caller prevents this failure without adding steps to the valid workflow.

Acceptance criteria:

• Publish and collect readiness and observation time as one coherent completed sample. Add deterministic concurrent success/failure regression coverage and preserve cancelled-caller behavior.
• Prove the regression fails on current main and passes with the smallest root-cause repair.
• Keep affected documentation consistent and deliver through reviewed, passing CI and an actual merge.

Success signal: the focused regression and valid-path controls pass within this delivery round. Size: small to medium.