Add exact rerank from authoritative base-table vectors

Parent: #19094
Related: #18676

## Summary
Implement exact rerank from authoritative base-table vectors with mandatory batched liveness arbitration and continuation.

## Scope
- Share MDT candidate generation and retain up to `maxRerankCandidates`.
- Batch candidate keys through RLI before authoritative vector fetch.
- Continue candidate scans when arbitration or positional reads invalidate finalists.
- Coalesce positional reads by file group and use stored row positions when valid.
- Handle MoR `rowPosition = -1` through the authoritative merged-read path.
- Score exact vectors and return final top-K.
- On stale index state or exhausted continuation/read budget, apply explicit `FAIL`, `WARN`, or exact-scan fallback policy; never silently present fewer than K as complete.

## Acceptance criteria
- Exact mode returns only authoritative live records.
- Deletes and cluster-changing updates cannot leak old postings into final results.
- Continuation retains the configured rerank candidate budget, not just K.
- Tests cover batched RLI arbitration, invalid locators, MoR log rows, duplicate/stale candidates, continuation, coalescing, and budget exhaustion.

## Dependencies
- #19102 candidate generation and shared continuation behavior.
- #19499 planner freshness contract for stale-policy decisions.

Start with dependency #19102 for candidate generation and shared continuation behavior, then read #19499 for the planner freshness contract and stale-policy decisions. Trace how batched RLI arbitration, authoritative reads, continuation, and reranking fit together; done means exact mode returns only live records and the listed tests cover invalidation, continuation, coalescing, and budget exhaustion.

Written by the indexing model from the issue text.