perf(table): load equality deletes lazily per scan task

[fallintoplace]

Summary

• Load equality deletes per scan task.
• Keep a shared, read-once cache for delete files and merged file combinations.
• Start scan workers when the iterator is consumed, so an unread iterator does no equality-delete I/O.
• Keep equality-field grouping and delete-set reuse.

The old path read every unique equality-delete file before returning the iterator. This change only reads files needed by tasks that are actually processed.

Benchmark

Setup: 10,000 scan tasks, 1,000 equality-delete files, 10 keys per file, 16 workers. Three runs with -benchtime=1s on an Apple M1 Pro.

case	time/op	bytes/op	allocs/op	opens/op
eager/all_tasks	12.8 to 13.7 ms	39.6 MB	299k	1,000
lazy/unread	0.45 to 0.49 ms	285 KB	11,023	0
lazy/first_task	0.50 to 0.54 ms	326 KB	11,298	1
lazy/ten_tasks	0.65 to 0.75 ms	666 KB	13,713	10
lazy/full_scan	13.1 to 14.6 ms	39.2 MB	287k	1,000

• Unread iterator: no equality-delete opens.
• Partial scans: I/O follows the tasks reached.
• Full scan: comparable runtime with fewer allocations.

Tests

• go test ./table -count=1
• go test -race ./table -count=1
• go vet ./table