refactor(planner): add explicit precompute physical plan

[zzylol]

Summary

• make the post-ASAP physical compiler emit one explicit PhysicalPlan with CollectorPlan, PrecomputePlan, and BackendPlan projections sharing one plan envelope
• model PrecomputePlan as config-driven materializations, with no PromQL job or scheduler-shaped fields
• publish the precompute projection through the backend streaming-config contract before BackendPlan and CollectorPlan activation
• delete the dead PrecomputeJob emitter/client, protobuf field, in-memory backend registry, and /api/v1/precompute* job endpoints that acknowledged work without scheduling it
• reserve the removed protobuf field number/name for wire safety

Execution model

ASAPPlanner post-ASAP IR
          ↓
      PhysicalPlan
       ├─ CollectorPlan
       ├─ PrecomputePlan
       └─ BackendPlan

PrecomputePlan.materializations drives metric/filter matching, accumulator selection, grouping, windows, and SketchStore writes through /api/v1/streaming-config. Query readouts and routing DAG remain exclusively in BackendPlan.

Validation

• cargo test -p control_plane — 678 lib + 28 API tests passed; 3 doc tests ignored
• cargo test -p data_plane drivers::query::servers::http --lib — 60 passed; 1 ignored
• cargo check -p control_plane -p data_plane
• git diff --check

Stacked on #468.