perf(table): index schemas by ID

[fallintoplace]

Summary

• Added schema ID indexes to metadata and MetadataBuilder.
• Keeps indexes current when schemas are added or removed.
• Uses copy-on-write for builder clones and built metadata.
• Added regression tests and benchmarks for lookup behavior, stale slices, cloning, and concurrent reads.

Benchmark

Command: go test ./table -run ^ -bench "Benchmark(CommonMetadataSchemaByID|MetadataBuilderGetSchemaByID)" -benchtime=50ms -count=1

• 8,192 schemas, builder last: about 9 ns/op indexed vs 7.4 us/op linear.
• 8,192 schemas, builder miss: about 150 ns/op indexed vs 7.8 us/op linear.
• 8,192 schemas, metadata last: about 200 ns/op indexed vs 7.7 us/op linear.
• Metadata lookups still return defensive schema copies.

Tests

• go test ./table -count=1
• go test -race ./table -run "SchemaIndex|SchemaByID|MetadataBuilderCloneCoversAllFields" -count=1
• go vet ./table
• golangci-lint run --timeout=10m