install: report actual errors for special-file targets

[Hashim1999164]

Summary

• let existing special-file destinations reach the normal replacement path instead of reporting them as missing
• add a FIFO regression test that verifies special targets are replaced safely

Fixes#9934

Validation

• cargo fmt --check
• cargo test --features install --no-default-features test_install_replaces_special_target (1 passed)
• cargo test --features install --no-default-features test_install:: (98 passed)
• cargo clippy --features install --no-default-features --all-targets -- -D warnings

[codspeed-hq]

Merging this PR will improve performance by 5.5%

⚠️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

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	du_wide_tree[(5000, 500)]	23 ms	21.8 ms	+5.5%

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 Hashim1999164:fix/install-special-file-target (1622492) with main (eae5c43)}

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:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)

[Hashim1999164]

The failing CI jobs look unrelated to this change (test_chgrp::test_reference environment/group assertion). Happy to rebase or re-run if maintainers want a fresh check.