perf(lease-read): cache LeaseProvider type assertion (deferred from #549)

[bootjp]

Issue

Coordinate.LeaseRead and groupLeaseRead perform c.engine.(raftengine.LeaseProvider) on every call. Each type assertion is ~1-3 ns; not a bottleneck but trivially cacheable.

Fix

• Add lp raftengine.LeaseProvider field on Coordinate and ShardGroup.
• Set once in NewCoordinatorWithEngine / NewShardedCoordinator (after the type assertion).
• LeaseRead checks c.lp != nil instead of asserting.

Acceptance

• Same observable behavior; LeaseRead benchmark shows reduced cycles per op.

References: PR #549 (deferred).