Run a top-K aggregation and write the ranked rows (Step 3 of #386)

[zipdoki]

Summary

A group can declare a top-K, but nothing computes one. The declaration sits in metadata and no ranked row is ever written. This runs the aggregation.

TopkAggregationHandler scans the source group with the declaration's ranges interpolated against the event, sums metric per (entity, dimension key), and writes rank rows keyed by rankSource(...) under the metric_desc index. When a declaration is a sliding window and refreshAfterMillis > 0, it also schedules a refresh
for the instant the bucket leaves the window. Consuming that refresh message is the next PR in the stack.

POST /aggregations/v1/aggregate drives it, taking a batch of edges:

{
  "items": [
    {
      "database": "graph",
      "table": "likes",
      "edge": {
        "source": "...",
        "target": "...",
        "...": "..."
      }
    }
  ]
}

{
  "items": [
    {
      "database": "graph",
      "table": "likes",
      "source": "...",
      "target": "...",
      "status": "OK",
      "error": null
    }
  ]
}

Status and error are reported per item, so one bad edge does not sink the rest of the batch.

Two bucket changes ride along because the window math depends on them. Bucket now has a test pinning how a date bucket rounds both a value and a range bound, which the refresh schedule reads directly, and its format precision is read in one place instead of being re-derived at each call site. Separately, rank keys escape | so
two rankings can never collide on one row.

Stacked on #493 — that needs to land first.

Part of #386.

Test plan

• ./gradlew :core:test --tests '*BucketTest*' — date bucket rounding for values and for range bounds
• ./gradlew :core:test --tests '*AggregationConstantsTest*' — rank key composition and | escaping
• ./gradlew :engine:test --tests '*RankingInputsTest*' — ranges interpolation, entity and dimension resolution
• ./gradlew :engine:test --tests '*TopkAggregationHandlerTest*' — aggregation over the source group, rank row writes, refresh scheduling
• ./gradlew :server:test --tests '*MetadataAggControllerE2ETest*' — POST /aggregations/v1/aggregate over HTTP
• ./gradlew spotlessKotlinCheck build — formatting and full build

AI Assistance

• This PR was written largely with AI assistance.
  • Tool / model: Claude Code (Opus 5)