perf(table): load deletion-vector Puffin groups lazily

[fallintoplace]

Summary

• Index deletion-vector ownership without opening Puffin files during scan setup.
• Load each Puffin group on first use and cache the decoded bitmaps with single-flight behavior.
• Keep the existing same-file grouping, range coalescing, and duplicate-DV validation.
• Start scan workers when the iterator is consumed, so an unread iterator does not load DV files.

Benchmark

Apple M1 Pro. 100 Puffin groups with 10 DV blobs per group.

case	Puffin opens/op	ns/op	B/op
eager all groups	100	2.75 ms	3.41 MB
lazy unread	0	0.23 ms	0.33 MB
lazy first group	1	0.30 ms	0.36 MB
lazy ten groups	10	0.62 ms	0.64 MB
lazy full scan	100	2.71 ms	3.40 MB

Tests

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