uucore: stop option parsing at first operand when POSIXLY_CORRECT is set

[krosci]

When POSIXLY_CORRECT is present in the environment, option processing must stop at the first non-option argument, treating all subsequent arguments as operands. This patch introduces argument preparation in uucore's clap localization and argument parsing entry points to insert the end-of-options separator before the first positional operand while preserving GNU compatibility for join and pr. Comprehensive unit and integration tests have been added across utilities to prevent regression.

Fixes #14621

[codspeed]

Merging this PR will degrade performance by 13.72%

⚠️ 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

❌ 2 regressed benchmarks
✅ 365 untouched benchmarks
⏩ 50 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	rm_single_file	105.1 ms	135.5 ms	-22.44%
❌	Simulation	factor_multiple_u64s[2]	68.3 ms	71.1 ms	-4.01%

Tip

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

---

_{Comparing krosci:fix/posixly-correct-arg-parsing (8a8664d) with main (b60091b)²}

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. ↩

2. No successful run was found on main (41c3451) during the generation of this report, so b60091b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!

[anastygnome]

Why ?

[anastygnome]

The perf regression is due to checking the envvar, nothing that we can avoid, sadly.