On-disk index cache for the Grid benchmark harness

[tlwillke]

This PR adds a deterministic on-disk index cache for the Grid benchmark harness and wires it in end-to-end so repeated runs can reuse previously-built graph indexes to save time.

Key changes

1. Introduced OnDiskGraphIndexCache (flat directory cache, one file per index) keyed by a stable signature derived from:

• dataset base name
• feature set (per-index; not dependent on the list of feature sets)
• build params (M, efConstruction, neighborOverflow, addHierarchy, refineFinalGraph)
• build compressor identity

2. Cache filenames are derived from the signature (sanitized for filesystem safety). This allows multiple cached indexes to coexist in a single flat cache directory without collisions.

3. Updated Grid.runOneGraph to treat caching per-index (per feature set):

• load cached indexes when present
• build only the missing ones and merge results
• keep non-cached builds writing into the temp work directory using the original graphN naming

4. Refactored buildOnDisk so it can write either:

• the original graph0..graphN temp files (cache disabled), or
• signature-named files in the cache directory (cache enabled), while preserving existing build behavior and minimizing churn.

5. Updated Bench, BenchYAML dataset files, and HelloVectorWorld

• disabled the index cache by default
• easy enabling of the cache if desired (useSavedIndexIfExists or enableIndexCache)

6. Improved logging so it’s obvious when a cached index is used vs built from scratch, and added a cache-enabled startup message pointing to the cache directory and how to reclaim disk.

Behavior

• Cache enabled: reuse cached indexes when signatures match; build only missing feature sets; artifacts persist in the cache directory.
• Cache disabled: no cache reads/writes; build into temp work directory and clean up as before. NOTE: cached files indeed persist and must be explicitly deleted.

Notes

• Signatures are per-index (per feature set) and include all build-defining params to prevent accidental reuse across incompatible builds.
• Filenames are sanitized to avoid filesystem issues from compressor IDs or other signature components.
• No impact on graph construction or other benchmarking performance
• Cleaned up a few minor bugs in the existing code
• Does not address disk use metrics (disk space or file count). They report 0 for cached indexes (a TODO)

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