Replace the cureent io.Copy with the io.CopyBuffer with pooled buffer

On Go 1.26+, *os.File implements WriterTo, which causes
io.CopyBuffer to bypass the provided pool buffer entirely.
Instead, File.WriteTo → genericWriteTo → io.Copy allocates
a fresh 32KB buffer on every call.

This test detects the bypass by counting allocations during
ioCopier copy operations with an *os.File source (which is
the common case when the last pipeline stage is a commandStage).

The test currently FAILS, demonstrating the problem.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

On Go 1.26+, *os.File implements WriterTo. When ioCopier's reader is an
*os.File (the common case for commandStage), io.CopyBuffer detects
WriterTo and calls File.WriteTo instead of using the provided pool
buffer. File.WriteTo's sendfile path fails (the dest is not a network
connection), so it falls back to genericWriteTo → io.Copy, which
allocates a fresh 32KB buffer on every call — defeating the sync.Pool
entirely.

Fix: wrap the reader in readerOnly{} to strip all interfaces except
Read, forcing io.CopyBuffer to use the pool buffer.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

When using WithStdout, the writer is wrapped in nopWriteCloser,
which hides the ReaderFrom interface of the underlying writer.
This prevents io.CopyBuffer from dispatching to ReadFrom for
potential zero-copy paths (e.g., when the destination is a
network connection or has a custom ReadFrom implementation).

This test currently FAILS, demonstrating the problem.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

When the pipeline's stdout is set via WithStdout, the writer is
wrapped in nopWriteCloser to add a no-op Close method. This
wrapper hides the ReaderFrom interface of the underlying writer,
preventing io.CopyBuffer from dispatching to it.

Fix: unwrap nopWriteCloser in ioCopier and call ReadFrom directly
when available. This enables zero-copy when the destination has a
meaningful ReadFrom (e.g., network connections, custom writers).
For the pipe-to-pipe *os.File case, File.ReadFrom's zero-copy
paths don't yet support pipe sources, so a follow-up commit adds
direct splice(2) for that case.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Check error returns from pw.Write, c.Start, c.Wait. Remove
redundant embedded field selectors (w.Buffer.String → w.String).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Enable sendfile for network destinations; fix pool bypass on Go 1.26+