perf(table): index snapshot removals with a set

[fallintoplace]

Summary

• Build one snapshot ID set in MetadataBuilder.RemoveSnapshots.
• Reuse it for the current snapshot guard and snapshot, log, statistics, and partition statistics filters.
• Replace repeated linear membership scans with map lookups while keeping the update payload and behavior unchanged.

Performance

In a microbenchmark on an Apple M1 Pro with 1,000 snapshot/log/statistics entries and 500 IDs to remove:

• Before: ~660 us/op
• After: ~63 us/op
• About 10.5x faster

The set allocation scales with the number of IDs being removed.

Tests

• go test ./table -run TestRemoveSnapshots|TestMetadataBuilder -count=1
• go test ./table -count=1