printf: avoid panic on numeric field width above u16::MAX

[MadeNavaneeth]

Fixes #13850.

printf '%65536d' 5 panicked with "Formatting argument out of range" because Rust's write! macro only accepts a width that fits in a u16. The boundary is exactly u16::MAX: %65535d worked, %65536d panicked (and any width up to the existing MAX_FORMAT_WIDTH guard of 1_000_000).

This replaces the write!-based padding in num_format::write_output with manual chunked space/zero padding (write_spaces/write_zeros), while keeping the existing check_width guard so widths above MAX_FORMAT_WIDTH still fail gracefully (matching GNU's behavior of erroring rather than hanging).

Verified: the new regression tests test_width_65536_does_not_panic and test_width_above_u16_max_succeeds pass, and the full test_printf suite (142 tests) passes with no regressions.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/csplit/csplit-heap is now passing!

[codspeed-hq]

Merging this PR will degrade performance by 10.16%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 3 regressed benchmarks
✅ 358 untouched benchmarks
⏩ 50 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	split_lines	9.2 ms	10.6 ms	-13.16%
❌	Simulation	split_numeric_suffix	9.7 ms	11.1 ms	-12.55%
❌	Simulation	split_bytes	644.5 µs	675.1 µs	-4.53%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing MadeNavaneeth:fix/printf-width-u16-max (dd3e4ae) with main (0d8310c)}

Footnotes

1. 50 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. ↩