perf(table): parallelize non-bulk PurgeFiles deletion

[fallintoplace]

Summary

• Parallelizes non-bulk PurgeFiles deletes with bounded workers.
• Keeps BulkRemovableIO as the first choice.
• Keeps missing-file deletes successful.
• Collects independent failures in deterministic input order.
• Stops queued work when the context is canceled.

Benchmark

Simulated 100µs per-file deletion:

• 1,000 files: 120ms with 1 worker, 30ms with 4 workers, 7.7ms with 16 workers.
• 10,000 files: 1.22s with 1 worker, 416ms with 4 workers, 76ms with 16 workers.

Tests

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