Decouple RLIBootstrapOperator from BucketAssigner

### Task Description

**What needs to be done:**

This is the structural change described in "Incremental Cache Maintenance": keep them as two chained operators (not merged) sharing one in-process RocksDB instance, so BucketAssigner's parallelism isn't capped by RLI file-group count. Likely the highest-risk PR — isolate operator-graph wiring changes here alone.

**Why this task is needed:**

### Task Type

Code improvement/refactoring

### Related Issues

**Parent feature issue:** (if applicable )
**Related issues:**
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