Joseph Doherty 38ae1f6bea feat: add topology-aware placement with tag enforcement (Gap 2.8) ...

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.

2026-02-25 08:59:18 -05:00

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