Persist structured checkpoint-I/O policy and runtime-image identity in startup telemetry

Motivation

The startup metrics added by #18608 provide model-loading timings, but effective checkpoint-I/O policy currently depends on parsing server logs. Missing logs or multiple primary/draft loading events can leave attribution incomplete. Metric rows also lack the launch-time runtime-image reference, making matched-image comparisons depend on external joins.

Proposed scope

• Forward the canonical finalized primary checkpoint-I/O status through existing startup metadata and /server_info, including requested/selected/activated/effective and fallback reason. Keep primary and draft attribution separate; unsupported/unobserved paths remain unknown.
• Prefer structured status in the CI collector, retaining explicitly labeled legacy-log fallback for older runtimes. Add separate timing and policy completeness indicators.
• Stamp the actual test-container launch reference at Kubernetes/Slurm launch sites, including architecture-correct runtime images, and persist it with provenance. Record an OCI digest only when actually supplied by a digest-pinned launch reference; tagged references do not prove a resolved digest.
• Add focused tests and document comparison semantics. No new CI stages or model launches.

Non-goals

No policy-allocation or loader behavior changes, native counterfactual eligibility probes, checkpoint fingerprints, slowest-rank timing/collectives, failed-start persistence, or launch-to-first-token instrumentation. Resolving the running digest for mutable image tags is separate infrastructure work.

Related: #18608, #17393. Independent of the 50/50 split in #19170.

Trace the existing startup metadata and /server_info paths, then inspect the CI collector and Kubernetes/Slurm launch sites. Confirm how primary and draft attribution, legacy-log fallback, completeness indicators, and launch-reference provenance are represented; add the focused tests and documentation needed to verify comparison semantics without adding CI stages or model launches.