* perf(vectordb): coalesce auto cuVS rebuilds during bulk ingest
Add an opt-in bulk-ingest maintenance scope that coalesces Auto cuVS background rebuilds across multiple write batches.
- defer derived GPU maintenance until the outermost bulk scope exits while keeping native writes and persistence visible per call
- harden the background worker against debounce, generation, shutdown, and stale-candidate races
- preserve suspension across index replacement and retire replaced workers
- wait for the final Auto GPU snapshot before vectordb_perf records search QPS
- document that the scope is non-transactional and only schedules readiness on exit
Auto cuVS and background rebuild remain disabled by default. Native CPU and remote backends use no-op hooks, so their existing behavior and dtype are unchanged.
* fix(vectordb): reject stale index replacements
* fix(vectordb): harden bulk rebuild lifecycle
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Co-authored-by: Yuanqing Zhao <2604121+yuanqingz@users.noreply.github.com>