perf(table): group deletion vector reads by Puffin file

[fallintoplace]

Summary

• group deletion vector loads by physical Puffin path
• open and decode each shared Puffin file footer once
• keep a fast path for one deletion vector
• keep concurrency across different Puffin files

Why

The DV writer stores one blob per data file in a shared Puffin file. The scanner was calling ReadDV once per blob, so 100 DVs in one Puffin opened and decoded the same file 100 times.

Benchmarks

BenchmarkReadAllDeletionVectorsSharedPuffin on Apple M1 Pro:

DVs	main	this PR	speedup
1	49.98 us	29.92 us	1.67x
10	672.21 us	97.03 us	6.93x
100	19.35 ms	558.17 us	34.66x

At 100 DVs, allocations drop from 19.3 MB and 125,737 allocs to 278 KB and 2,793 allocs. The shared Puffin file is opened once instead of 100 times.

Tests

• go test ./...
• go test -race ./table/dv ./table -run "TestReadDV|TestReadDVs|TestReadAllDeletionVectors" -count=1
• go vet ./...