* feat(balance): add replica placement validation for volume moves
When the volume balance detection proposes moving a volume, validate
that the move does not violate the volume's replication policy (e.g.,
ReplicaPlacement=010 requires replicas on different racks). If the
preferred destination violates the policy, fall back to score-based
planning; if that also violates, skip the volume entirely.
- Add ReplicaLocation type and VolumeReplicaMap to ClusterInfo
- Build replica map from all volumes before collection filtering
- Port placement validation logic from command_volume_fix_replication.go
- Thread replica map through collectVolumeMetrics call chain
- Add IsGoodMove check in createBalanceTask before destination use
* address PR review: extract validation closure, add defensive checks
- Extract validateMove closure to eliminate duplicated ReplicaLocation
construction and IsGoodMove calls
- Add defensive check for empty replica map entries (len(replicas) == 0)
- Add bounds check for int-to-byte cast on ExpectedReplicas (0-255)
* address nitpick: rp test helper accepts *testing.T and fails on error
Prevents silent failures from typos in replica placement codes.
* address review: add composite replica placement tests (011, 110)
Test multi-constraint placement policies where both rack and DC
rules must be satisfied simultaneously.
* address review: use struct keys instead of string concatenation
Replace string-concatenated map keys with typed rackKey/nodeKey
structs to eliminate allocations and avoid ambiguity if IDs
contain spaces.
* address review: simplify bounds check, log fallback error, guard source
- Remove unreachable ExpectedReplicas < 0 branch (outer condition
already guarantees > 0), fold bounds check into single condition
- Log error from planBalanceDestination in replica validation fallback
- Return false from IsGoodMove when sourceNodeID not found in
existing replicas (inconsistent cluster state)
* address review: use slices.Contains instead of hand-rolled helpers
Replace isAmongDC and isAmongRack with slices.Contains from the
standard library, reducing boilerplate.
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