sort: don't accept leading '+' in numeric (-n) sort

[c-tonneslan]

Rust's f64 parser happily accepts +1 as 1.0, so the whole-line numeric fast path in sort -n was treating +1, +10, +2 as numbers and sorting them numerically. GNU sort -n doesn't accept a leading + (only -g does), so those lines should compare lexicographically and stay in their original order.

Reproduces on main:

$ printf '+1\n+10\n+2\n' | target/debug/sort -n
+1
+2
+10
$ printf '+1\n+10\n+2\n' | /usr/bin/sort -n
+1
+10
+2

Fix: exclude inputs containing + from the fast path so they fall through to the regular comparator, which already rejects + as a sign.

Closes #10315.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tee/tee is no longer failing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!