wc: optimize -c when reading from stdin regular file

[venoosoo]

fixes #11610

This PR adds the missing metadata optimization for wc -c (and --bytes) when input comes from stdin redirection (wc -c < file).

GNU coreutils uses fstat() to get the file size directly in this case, avoiding reading the entire file. uutils previously always read the data, making it very slow on large files.

Changes

• Added try_get_stdin_size() helper using rustix::fs::fstat (Unix)
• Integrated it into the existing fast path for bytes-only mode
• Added test for the optimization
• Falls back safely for pipes, terminals, character devices, etc.

also didnt really know windows so i just added todo to windows implementation

[oech3]

This should be part of benchmark

[oech3]

But I don't know how to use stdin at CodSpeed. Maybe, you can just drop the test.

[venoosoo]

alright

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/wc/wc-proc. tests/wc/wc-proc is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[oech3]

FAIL: tests/wc/wc-proc