feat(metrics): add Micrometer gauge for quarantined hypergraph vertices

[forgespectrayan]

Context

During the implementation of Issue #946 (Hypergraph Vertex Quarantine, ADR-0082 Phase 2.1), the QuarantineRegistry was introduced with a quarantinedCount() accessor suitable for Micrometer gauge binding.

However, MeterRegistry is not currently wired into the IndexReconcileEngine or IndexPlaneCoordinator. As agreed during planning (#946), the Micrometer gauge is deferred to a dedicated follow-up.

Scope

1. Bind a Micrometer gauge spector.index.hypergraph.quarantined.vertices to QuarantineRegistry.quarantinedCount() in DefaultSpectorMemory or QuartzMemoryScheduler where MeterRegistry is available.
2. Add optional MeterRegistry parameter to IndexPlaneCoordinator or IndexReconcileEngine.
3. Tag the gauge with namespace for multi-tenant observability.

References

• Issue feat(memory): implement Hypergraph vertex quarantine in IndexReconcileEngine (ADR-0082 Phase 2.1) #946 (parent)
• PR feat: Index Plane Lifecycle and Derived View Reconciliation (ADR-0082, #811, #428) #945 (feat/adr-0082-index-plane-lifecycle)
• ADR-0082: docs/adr/0082-index-plane-lifecycle-and-derived-view-reconciliation.md