perf(table): skip metadata builds when Transaction.apply has no new requirements

[fallintoplace]

Summary

• Lazy metadata builds: Transaction.apply now waits to build immutable metadata until a new requirement needs validation.
• No-op requirement paths: Empty requirement lists and duplicate requirements skip MetadataBuilder.Build() while keeping conflict checks.
• Atomic staging: The existing cloned builder and publish behavior stay unchanged.

Benchmark

Apple M1 Pro, Go 1.26.3, 10,000 snapshots:

• No requirements: about 1.50 MB / 65 allocs to 1.21 MB / 14 allocs
• Duplicate requirement: about 1.50 MB / 70 allocs to 1.21 MB / 19 allocs
• New requirement: about 1.51 MB / 77 allocs before and after

Tests

• go test -timeout=5m across all packages except io/gocloud
• go test -race -timeout=5m . ./table/...
• golangci-lint run --timeout=10m