CASSANDRA-21550 Phase1 BIG splitter only

[clohfink]

Summary

This adds an opt-in --zero-copy path to sstablesplit for compressed BIG-format SSTables.

Instead of deserializing and recompressing partitions, the splitter retains existing compression chunks and rebuilds the child SSTables’ indexes and metadata. On supported Linux filesystems it uses byte-range reflinks to share data extents; otherwise it copies the compressed bytes without rewriting rows.

The implementation:

• Supports compressed BIG pa-or-later SSTables and writes pb children.
• Requires storage_compatibility_mode: NONE.
• Rejects uncompressed SSTables, SSTable-attached indexes, and unsupported components.
• Persists the first indexed partition position so readers, streaming, scrub, verification, loading, and upgrades correctly handle retained chunk prefixes.
• Treats the requested size as a maximum unless a single partition’s compression-chunk span exceeds it.
• Falls back safely to copying when reflinks are unavailable.

Commits

1. ce328da5550a129d1d5d8377afd16da458129514 — Add zero-copy splitting for compressed BIG SSTables

   Contains the production implementation and sstablesplit documentation, including the splitter, index rebuilding, reflink support, metadata changes, and updates to affected SSTable consumers.

2. c07e4931783181e87054484b7b51626b21f6300b — Add tests for zero-copy SSTable splitting

   Contains only tests covering splitting and sizing, reflink behavior and fallback, metadata and version compatibility, corruption handling, streaming, scrub, verification, upgrades, memory-mapped regions, and the command-line tool.