XY-708: Operator degradation P1: preserve typed root causes in degraded readback

[yvette-carlisle]

Summary

• Add typed operator-local readback_root_cause diagnostics for degraded retained PR readback while preserving public-safe warning reasons.
• Surface the diagnostic in status JSON/text and dashboard retained-lane facts.
• Update runtime/operator docs and regression coverage for distinct root-cause classes.

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make test