Avoid whole-loading index sidecars during lookups

  Refactor index query paths to operate on sidecar handles instead of
  materializing payload arrays in memory.

  - stream full exact lookups from sidecars for both compact and run-bounded paths
  - stream ordered full reuse with on-the-fly merge of base and incremental runs
  - remove light and medium exact-query fallbacks that whole-loaded payload sidecars
  - cache in-memory sidecars as compressed handles instead of NumPy arrays
  - add handle-based summary/boundary readers for query planning and navigation
  - fix in-memory cache cleanup so stale sidecar handles do not survive array finalization
  - keep light lossy chunk pruning conservative to avoid dropping boundary matches
  - add regression tests guarding against _load_array_sidecar / _load_full_arrays
    in exact and ordered query execution

Author: FrancescAlted