perf(codec): precompute DataFile Avro field indexes

[fallintoplace]

What changed

• Precompute the indexes of dataFile fields carrying an avro tag once during package initialization.
• Reuse those indexes when cloning a DataFile for MarshalAvroEntry.
• Keep tag discovery reflection-based so dataFile remains the single source of truth for the wire shape.
• Add a coverage test and focused benchmarks.

Why

MarshalAvroEntry needs a shallow copy because dataFile contains synchronization state. The old clone helper scanned every struct field and looked up its Avro tag for every encoded manifest entry. The new helper keeps the same copy semantics and removes that repeated metadata lookup.

Benchmark

Apple M1 Pro, Go 1.26.3, -cpu=1, median of 8 runs.

Benchmark	Before	After	Change
CloneDataFileAvroFields	2.00 us	0.46 us	-77%
MarshalAvroEntry/v1	18.02 us	15.88 us	-12%
MarshalAvroEntry/v2	17.53 us	15.92 us	-9%
MarshalAvroEntry/v3	18.51 us	15.77 us	-15%

Allocation counts and bytes per operation are unchanged:

• Clone: 320 B/op, 1 alloc/op
• Marshal v1: 13,376 B/op, 144 allocs/op
• Marshal v2/v3: 13,360 B/op, 144 allocs/op

Command:

go test . -run '^$' -bench '^(BenchmarkCloneDataFileAvroFields|BenchmarkMarshalAvroEntry)$' -benchmem -benchtime=500ms -count=8 -cpu=1

Testing

• go test .
• go test -race .
• go vet .
• golangci-lint run --timeout=10m
• All packages other than the metadata-endpoint-dependent io/gocloud package pass with go test.