docs: add SAFETY comments to all unsafe blocks in shuffle spark_unsafe module

[andygrove]

Summary

• Add // SAFETY: comments to all 25 existing unsafe blocks in the spark_unsafe shuffle module (row.rs, list.rs, map.rs)
• Add a # Safety documentation section to the SparkUnsafeObject trait documenting invariants for implementors
• No functional changes

All unsafe in this module falls into one category: reading/writing JVM-allocated Spark UnsafeRow/UnsafeArray/UnsafeMap memory via raw pointers received through JNI. The safety invariants are:

• Pointers come from the JVM and remain valid for the batch lifetime (guaranteed by JNI pinning)
• Memory layout follows Spark's documented UnsafeRow/UnsafeArray format
• Index bounds are enforced by the caller (schema-driven iteration)

Test plan

• Documentation-only change, no functional impact
• Verified cargo check passes

🤖 Generated with Claude Code

[wForget]

Thanks @andygrove