tee: avoid 0fill & heap alloc, remove NamedReader

[oech3]

• Remove NamedReader which is mere Stdin.
• Stop 0-filling large buffer & drop outdated comment about its size.
• Dedup 2 read loop.

[codspeed-hq]

Merging this PR will improve performance by 37.19%

⚡ 2 improved benchmarks
✅ 317 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tee_stdin_file[10000000]	233.2 µs	180 µs	+29.59%
⚡	Memory	tee_stdin_file[10000000]	102.7 KB	70.7 KB	+45.24%

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-uninit (97df98c) with main (0763c0b)}

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/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!