Fix: publish ALB metadata as atomic generations to avoid partial reads (#78)

The metadata collector cleared the alb-prewarm-metadata table and then rebuilt it item by item, while the LCU modification function scanned the same table with no gating. A modification run overlapping a collection run could observe an empty or partially rebuilt table, and would either exit early via “No ALB data found in DynamoDB table” or apply capacity changes to a subset of the tagged ALBs.

The collector now tags each run’s items with a unique Generation, commits the run with a single PutItem on a control item, and only then removes older generations using conditional deletes. The modification function reads the control item and processes only the committed generation, so an in-progress collection is never visible to it.

Two additional guards: the collector is limited to one concurrent execution, and a run that yields no data because of errors keeps the previously committed generation rather than publishing an empty snapshot.

Tables with no control item fall back to reading every item, so the first collector run after an upgrade is not blocked.