Clarifies WAL mode tradeoffs in docs and comments. - #150

Summary

Clarifies WAL mode guidance and enum comments so the durability/throughput tradeoffs are described accurately.

• Positions AsyncCompressed as the recommended high-throughput persistent default
• Describes Sync as the plain synchronous WAL write path for applications that specifically need that behavior
• Explains that SyncCompressed uses compressed-block/tail behavior and has weaker crash durability than Sync
• Updates README, docs, and NuGet README guidance to use the same terminology
• Removes an unused ZoneTreeOptions helper
• Improves atomic operations docs.