Phase 5 — Advanced Capabilities: Mode A coordination and HPA custom metrics
## Changes
- Add Mode A coordinator for rendezvous hashing (mode_a_coordinator.rs)
- Update task pruner to support Mode A partitioned ownership
- Add task store batch methods for Mode A pruning (list_terminal_tasks_batch, delete_tasks_batch)
- Add HPA custom metrics support (targetRequestsInFlight, targetBackgroundQueueDepth)
- Update Helm chart HPA template with custom metrics
- Update values.schema.json for HPA custom metrics fields
## Mode A Coordination
Implements rendezvous hashing for shard-partitioned ownership across pods.
Applies to anti-entropy, settings drift check, task pruner, TTL sweeper, and canary runner.
## HPA Custom Metrics
Adds support for autoscaling on custom metrics:
- miroir_requests_in_flight (per-pod metric)
- miroir_background_queue_depth (global metric)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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