Summary

Re-enable the sharded FuseFilter existence-filter format by setting ExistenceFilterVersion back from 1 to 2 in db/recsplit/recsplit.go. This is the switch that #21164 turned off after #20644 introduced the format. New index files (.kvi/.efi) will again embed the sharded BinaryFuse filter (256 shards by keyHash>>56), which cuts existence-filter build-time RAM ~256× (the motivation was bloatnet 3B-key .efi builds).

Why it's safe

• Read path is version-aware. db/recsplit/index.go reads the embedded dataStructureVersion per file and dispatches to v0/v1/v2 readers, so existing v1 files stay readable and new v2 files are read by the matching case 2 path.
• Accessor/existence files are built locally, not seeded. SeedableV3Extensions() publishes only .kv/.v/.ef/.ap; .kvi/.efi/.kvei are rebuilt locally via BuildMissedAccessors, so this does not affect the shared snapshot set.

Test plan

• CI green