perf(manifest): project scan columns during reads

[fallintoplace]

Summary

• Projects manifest data-file reads to the fields needed for scan tasks.
• Keeps metric maps only when row pruning or positional-delete matching needs them.
• Drops transient metric maps before local Arrow tasks retain the files.
• Keeps PlanFiles() on the existing full-metadata path for callers that inspect task stats.
• Caches projected Avro schemas by writer schema and projection.

Benchmark

Command: go test . -run=^\$ -bench BenchmarkManifestEntryProjection -benchtime=1x -count=1

Apple M1 Pro, 10,000 entries per case:

Stats fields	Full bytes	Scan bytes	Full allocs	Scan allocs
10	32.6 MB	15.6 MB	443,928	65,797
100	260.2 MB	100.5 MB	2,257,763	77,930
1,000	2.93 GB	1.45 GB	20,512,582	332,865

Validation

• go test . ./table -count=1
• go test ./... -run=^\$ -count=1
• go test -race ./table -run=TestOpenManifestWithProjectionDropsStatsAfterFiltering -count=1
• go vet ./...
• golangci-lint v2.12.2