Extends PlacementEngine.SelectPeerGroup with three new capabilities ported
from jetstream_cluster.go:7212 selectPeerGroup:
- JetStreamUniqueTag enforcement: PlacementPolicy.UniqueTag (e.g. "az")
ensures no two replicas share the same value for a tag with that prefix,
matching Go's uniqueTagPrefix / checkUniqueTag logic.
- MaxAssetsPerPeer HA limit: peers at or above their asset ceiling are
deprioritised (moved to fallback), not hard-excluded, so selection still
succeeds when no preferred peers remain.
- Weighted scoring: candidates sorted by
score = AvailableStorage - (CurrentAssets * assetCostWeight)
(DefaultAssetCostWeight = 1 GiB) replacing the raw-storage sort, with a
custom weight parameter for testing.
10 new tests in TopologyPlacementTests.cs cover all three features and their
edge cases. All 30 PlacementEngine tests continue to pass.
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