perf(table): skip manifests with no live entries

[fallintoplace]

Summary

• Skip data and delete manifests when both added_files_count and existing_files_count are known to be zero.
• Keep manifests with unknown counts for safety.
• This avoids opening manifests that only contain deleted entries.
• Added scanner regression coverage and a planning benchmark.

Benchmark

Command: go test -run '^$' -bench '^BenchmarkPlanFilesSkipsKnownEmptyManifests$' -benchtime=2s ./table

Apple M1 Pro, 256 manifests, 64 entries per manifest.

Case	Before	After
All empty	65.0 ms/op, 256 manifest opens/op	0.31 ms/op, 0 manifest opens/op
10% live	65.6 ms/op, 256 manifest opens/op	10.7 ms/op, 26 manifest opens/op

Allocations also dropped from 81.0 MB to 0.32 MB per operation for the all-empty case, and from 81.6 MB to 9.1 MB for the mixed case.

Checks

• go test ./table -count=1
• go test -race ./table -count=1
• golangci-lint run --timeout=10m