Limit rollout metadata reads to headers

[copyberry]

Limit rollout metadata reads to headers

Why

Callers that only need session metadata should not scan an entire rollout or fail on an unreadable tail after valid metadata.

What changed

• Read session metadata directly from the rollout line reader, stopping as soon as it is found for both plain and compressed rollouts while preserving pre-header and validation behavior.
• Bound the doctor thread inventory scan to the first 64 non-empty header lines. Prefer the metadata thread ID and retain the validated filename fallback for legacy rollouts without metadata.

Testing

• Cover invalid UTF-8 tails, compressed rollouts, pre-header records, metadata and filename ID mismatches, the scan boundary, legacy fallback, and unusable headers.

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