perf(table): fetch orphan-cleanup manifest lists concurrently

[fallintoplace]

Summary

• Reads snapshot manifest lists concurrently using the existing cleanup concurrency limit.
• Stores each snapshot result separately, then folds results in snapshot order before deduplicating manifests.
• Calls metadata.Snapshots() once and adds regression and benchmark coverage.

Benchmark

Representative -benchtime=1x results with a 1 ms artificial Open delay on an Apple M1 Pro:

Snapshots	Concurrency 1	Concurrency 16
16	22.9 ms	3.4 ms
128	171.2 ms	13.3 ms
1,024	1.40 s	99.6 ms

Tests

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