tee: add splice() and tee() zero-copy fast-path

[oech3]

Closes #11466 .

> tee </tmp/huge |pv >/dev/null
^C58GiB 0:00:01 [2.58GiB/s] 
> tee-splice </tmp/huge |pv >/dev/null
^C.6GiB 0:00:02 [38.4GiB/s]

[codspeed-hq]

Merging this PR will improve performance by 6.42%

⚡ 1 improved benchmark
✅ 322 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tee_stdin_file[10000000]	179.7 µs	168.8 µs	+6.42%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing oech3:tee-splice-m (9bb5588) with main (0011d54)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[oech3]

@sylvestre @cakebaker Would you review this?

[oech3]

@sylvestre Would you review this?