feat(realtime): add hash-based offset indexes with data file rolling

[lxy-9602]

Purpose

Linked issue: #158

• Add pool-backed HashMap and DenseHashTable utilities adapted from Havenask.
• Store key hashes instead of original keys, with lazy disk lookup through a shared CacheManager.
• Generate one .offset sidecar per rolled data file, including lifecycle and abort cleanup.

Tests

• UT coverage for hashing, containers, cached reads, rolling and cleanup.
• IT coverage for reads before/after prepare and refresh, plus restart recovery.

API and Format

No public API changes. Replace the previous .offset layout with version 1 containing key-hash-to-offset entries. Previous files are incompatible; hash collisions are not resolved.

Documentation

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