Perf: use Stream.CopyTo and zero-copy MemoryPackagePart reads

[ken-swyfft]

Summary

• StreamHelper.CopyStream: Replaced manual 1KB buffer loop with Stream.CopyTo() which uses an 80KB internal buffer — available since .NET 4.0
• MemoryPackagePart.GetInputStreamImpl: Eliminated full-stream copy by wrapping the existing MemoryStream buffer in a read-only view via GetBuffer() (zero-copy)

Benchmark results (test-performance.xlsx, 17MB, 5 sheets)

Benchmark	Before	After	Change
Write	2,843 ms / 103,176 KB	2,752 ms / 73,263 KB	~3% faster / 29% less memory
Load (full)	14,386 ms	13,932 ms	~3% faster
Load (lazy)	1,738 ms	1,804 ms	within noise

The Write benchmark shows the most significant improvement: 29% reduction in memory allocations (~30MB less per operation) from eliminating redundant MemoryStream copies during package writes.

Test plan

• All 4,616 tests pass on net8.0 (main + ooxml + openxml4net)
• All 4,619 tests pass on net472 (main + ooxml + openxml4net)
• BenchmarkDotNet comparison against master baseline

🤖 Generated with Claude Code

[ken-swyfft]

Code Review

Overall: Approve. Both changes are correct and safe across all target frameworks.

StreamHelper.CopyStream → Stream.CopyTo()

Straightforward replacement. Internal buffer goes from 1KB to 80KB (the CopyTo default) — strictly better for the two remaining call sites (thumbnail operations). Dead totalRead variable correctly eliminated.

MemoryPackagePart.GetInputStreamImpl — zero-copy buffer view

The change from a full defensive copy to a read-only MemoryStream view over GetBuffer() is safe. I verified:

1. data is always constructed via the parameterless new MemoryStream() (resizable), so GetBuffer() won't throw UnauthorizedAccessException.
2. The returned stream is writable: false, preventing accidental mutation.
3. All callers of GetInputStream() only read from the stream (XML parsing, IOUtils.ToByteArray, IOUtils.Copy, Read loops).
4. MemoryPackagePartOutputStream always replaces data with new MemoryStream(), so outstanding views referencing the old buffer remain valid and are not corrupted.

Minor suggestion

Consider adding a brief comment on the GetBuffer() line explaining the safety invariants for future maintainers, e.g.:

// Return a read-only, non-copying view over the internal buffer.
// Safe because callers only read, and GetOutputStreamImpl() replaces data entirely.
return new MemoryStream(data.GetBuffer(), 0, (int)data.Length, writable: false);

This makes the preconditions explicit so a future contributor doesn't accidentally break them (e.g., by constructing data from a byte[], which would make GetBuffer() throw).

Also noted

• ZipPartMarshaller.Marshall (lines 54-67) has a similar manual buffer-copy loop (8KB buffer) that could benefit from Stream.CopyTo as a follow-up.
• Pre-existing potential null-ref in MemoryPackagePart.Load() if called before GetOutputStreamImpl() — appears to be dead code, not introduced by this PR.